Medivh is my favorite hero in the game. His role in WC3 was mysterious and interesting, and his kit in HotS can be extremely satisfying and one of the more powerful in the game under the right conditions. But, I'm under no illusion that the uniqueness of his kit and abilities make his contributions to victory in your average game difficult to capitalize on. Outside of organized play with a team familiar with working together, playing Medivh is a labor of love, not a safe bet.

Pros:Mobility for the whole team, anti-dive / anti-burst, strong crowd control potential, stealing mercs from enemy team potential.
Cons:Very difficult to play well, slow wave clear before level 20, can't get camps, crowd control unreliable, has an important quest with stacks that get lost on death until completion.

Look to take Medivh when you're in a group of pretty competent players and can fully capitalize on the mobility he provides for the whole team, or, when the enemy team has strong initiation and burst that you can counter outright with Force of Will or his ults Poly Bomb and Ley Line Seal

Level 1:

Winds of Celerity is the best choice in most environments at level one. I have taken Raven's Intellect on maps where I expected extended team fighting and thought I'd need just a bit more mana regen to keep people shielded, but every time I do I always, always wish I had taken Winds of Celerity when the game continues. Especially if it gets to level 20 and I finally have Guardian of Trisifal, but I'm left slowly flying from lane to lane to make use of it. Between the two bird talents, movement speed is the most beneficial throughout the whole game and really adventitious in the late game.

Portal Mastery is a valid choice basically only in a game where you expect a high level of team competency. You can find marginal cases where staying a bird while providing a portals for your team allows for some strong position play. But when you're playing Medivh among randoms, just assume they will not use your portals. Provide them, use them to the advantage of your team, not only yourself, but never expect them to use them.

Level 3:

Raven Familiar provides you, and anybody else who uses your portal, with free extra focused damage on the next enemy hero they auto attack. Every time you go through the portal. And when poking during a fight, you can go through a portal two or three times before it expires. As Medivh, part of what you provide is damage.

Mage Armor really just doesn't provide enough protective benefit to be worth it. Even when used to its best possible extent, IE: when your team as a whole goes through an aggressively placed portal to attack the enemy while the 4 seconds of armor are up, they could have gone through with a bird to help actually secure those aggressive kills. Dust of Appearance only helps provide intel, and in a pretty superfluous way since you're already an invulnerable bird by default. What's a few extra inches or a spot reveal outside of specifically stopping a Valeera initiation or to more easily observe a hostile camp for potential theft with Ley Line Seal.

Level 7:

Arcane Explosion is in my opinion the strongest pick in this tier because it plays into the moment by moment mode of operation of Medivh as a whole, and it only grows more potent with the level 13 talent Circle of Protection. It turns one of the strongest defensive basic abilities in the game into an offensive tool that comes into play right at a moment when somebody vulnerable on your team is going to be shielded, healed, and subsequently deal good damage back to their attacker, making for very good trades in health.

I have played a lot with Mystic Assault and Force of Magic and neither of them flow into Medivh's play style very fluidly. With Force of Magic while the benefit is relatively easy to trigger (blocking x amount of damage with a shield), you then naturally feel compelled to be more aggressive while the buff is active, and at the exact time that your shield is going to be on cooldown. Mystic Assault as well is easy enough to trigger, but it's actually very rare that you will be lacing in auto-attacks in between casts of Arcane Rift, and even less rare that you'll be able to do that for an extended amount of time. Especially if you've completed the Arcane Rift quest. It's maximal use is when you turn Medivh into a quasi stutter-stepping ranged auto-attacker, and I find that demand of attention to positioning and accuracy distracts you from watching for other opportunities to shield friends or making an important play with your ultimates.

Level 10:

Ley Line Seal is my default choice because of it's versatility and ability to remove important but hard to reach targets from a team fight for a fairly long time. Poly Bomb is a great crowd control option too, but in a more counter-initiation way I'll explain below. But Ley Line Seal has the ability to entirely remove half of an enemy team for three seconds. It's imperative you choose your targets for the seal properly, as with any such powers, like Zeratul's Void Prison. You can just as easily completely screw your team if used wrong.

Typically you always want to remove the enemy team's back line with Ley Line Seal. Use it to remove the enemy healer and ranged DPS just after the enemy melee dive into your team and will need heals soon. Do NOT use it on an enemy that's been targeted by Pyroblast or on a The Butcher that's just charged into his target. In the The Butcher situation, make the judgement whether his target will survive the assault by the Butcher if you instead seal up the rest of Butcher's team. Butch may have charged in thinking he would have them to back him up.

Speaking of The Butcher and other strong engagers or sticky melee damage dealers, this is where you start considering taking Poly Bomb over the other ult. When you feel it's more important to shut down those incredibly strong windows of power from a single (usually melee) target, take Poly Bomb. Abilities you should be looking for to counter with it are things such as Dragonblade, Dragonqueen, Illidan before he can escape, Tracer before she can escape, Jug of 1,000 Cups, Holy Word: Salvation, Greymane as he goes in for the kill.

Level 13:

Circle of Protection is the natural choice given our level 7 choice of Arcane Explosion. The reasons why Arcane Explosion is good just get multiplied when you can protect multiple people at once. And, it's usually when team mates have gotten clustered up that they're just begging to be completely whammied by some enemy wombo-combo. This negates all of it, on all the poor saps caught right in the middle of it. And turns all that damage back on their attackers.

Again, I have used Enduring Will and Reabsorption extensively, at one point I preferred Enduring Will. Having a 1.5 second invulnerability shield on a 4 second cooldown is exceptionally annoying to somebody like Li-Ming. But, in practice, it's really those oopsie moments where your whole team WOULD otherwise have been obliterated by an enemy alpha strike while they were grouped up that makes Circle of Protection far and away the most reliable useful talent at this tier. And the same reasons go for why it beats out Reabsorption.

Level 16:

Arcane Charge is just the most damage. Even in a fairly slow game, by now you should have completed your Arcane Rift quest. That means with the extra 15% damage from Arcane Charge you're throwing out about 450 damage to any hero hit by Arcane Rift every second. The damage adds up very quickly and it's safe damage, no goofy auto attacking or shielding for buffs required. Just more Medivh, more better.

Temporal Flux is more of a gimmick. Basically if you've gone Poly Bomb, and you hit at least three enemy heroes with three Arcane Rifts, you can cast Poly Bomb again immediately. While fun, that's not typically going to make the situation better. Or, rather, it may be somewhat of a "win more" gimmick. The effectiveness of Poly Bomb or Ley Line Seal is already there, built into them. Having them on unreliably shorter cooldowns pales in comparison to the raw damage you are guaranteed to enjoy from taking Arcane Charge

Level 20:

Guardian of Tirisfal is what makes Medivh a real hero. If you've ever watched any of my streams while playing Medivh, you are guaranteed to have heard me lament about how this talent is so integral to Medivh as a hero. That if it were up to me, I would put it at talent level 16 rather than 20. Most of the time you will feel like getting all the way to 20 is too late for its benefits to kick in. By that point you might really feel like a Guardian of your core, since it's enabling you to frantically push back endlessly encroaching lanes. But even if temo-wise I think this talent is a much better candidate for level 16, it's at 20, and it's by far the best choice for your average game.

Reliability is the name of the game. Dust of Disappearance? Maybe in a coordinated strike on a coordinated team. Not for pick-up pub games. 15% extra Spell Power from Arcane Intellect? A substantial amount of extra damage, but probably isn't going to turn fights around if they've already been going poorly enough that you're level 20. And like before, enhancing Poly Bomb or Ley Line Seal is a little superfluous when their core usefulness is already provided without enhancement. There are better marginal reasons to take either ult talent at 20, but by default, by level 20, probably by level 16, you will have wished Medivh could AT LEAST clear minion waves quickly between participating in fights. Now he can.

Take advantage of your practically invulnerable split-push potential and create opportunities to turn the pressure you've been under back on the enemy team. Hopefully get the enemy team to split up for the next objective because they finally have some cats near core they have to deal with. Don't neglect your team mates or objective fights to go full autistic split pusher, but you have the tools you need to contribute the best you can in the most reliable ways.