Detachable Box Magazine: is in my opinion the strongest talent for this level. The bonus damage is nice, but the real reason for this is the active. You can use it right after firing 1 dart to fire 4 almost all at once. Combined with Biotic Grenade, this heals over 3000 health in 1 burst at level 20. Just don't miss them all, it is REALLY embarrassing.

Piercing Darts: can be OK, but requires you to use your sleep dart more then might be wise. Keep in mind that it is your only survivabilty tool. The reward is really nice however, as it lets you stay further away from the fight.

Grenade Calibration: is probably the worst talent for this level. With double support, it can be nice, but the first reward just doesn't do much of anything, as the grenade doesn't do any real damage to start with.

Overall, i would say that Detachable Box Magazine is the strongest for this tier. Be careful to not take to much damage to get stacks though.

Aim down Sights (ADS): this is a really strong ability, as it lets you poke more freely. There is a cooldown between activating it and deactivating it however, so don't get caught with it on, 20% less move speed sucks. It also has nice synergy with both Detachable Box Magazine and Piercing Dart at level 1.

Air Strike: this lets you throw a grenade with massive range, but you rarely want to be that far away in the first place. The 4 seconds off the cooldown is nice, but just not enough.

Overdose: only useful if you are having a hard time getting stacks for Detachable Box Magazine, otherwise it does very little.

Overall, ADS is a clear winner. The fact that you can toggle it makes it too strong. The extra sight can also help in a few scenarios.

This tier really depends on the enemy team.
If they have lots of mages, Mind Numbing Agent is a godsend. Sorry Li-Ming, you only deal 25% damage for 5 seconds.

If they rely more on auto attacks, AND you can hit sleep dart, Temporary Blindness can be really annoying. If nobody wakes them up (so like never), it removes them from the fight for 5 seconds. However, sleep dart has a really long cooldown and a small hitbox, so only take this if you are confident that you can hit it.

Debilitating Dart: is good versus most assassins. Similar to Shrink Ray other people get, this is a skill shot. However, you are playing Ana. You really should be able to hit those. This can also save you from certain situations.

Overall, take what you fell most comfortable in any given match.

Both of these are VERY powerful, so don't be afraid to try them both.

Eye Of Horus: lets you influence a teamfight from anywhere. You can also use it to finish off low health enemies, or save an ally. Watch out for the following 3 things: first, people blocking what you want to hit. Wasting shots on the full health friendly Murky while Genji dies sucks. Second, structures. Make sure that the side walls aren't in your way. Third, you are defenseless while shooting. Use it from a safe spot.

Nano Boost: can win the game for your team. Give Kel'thuzad this and he can kill everyone. Make sure the person you fire knows it's coming and can benefit from it. Valeera is a bad target, as she doesn't regenerate energy fast enough to spam her spells.

Another tier that depends on the enemy team.

Speed Serum makes for an excellent default. The extra move speed won't be that noticeable, but it lets your team get the positional advantage more often then not.

Purifying Darts should be taken if the enemy has lots of slows/roots, like Arthas or Jaina. Not much else to say here.

Smelling Salts: again, not much to say other then this counters stuns. As a side note, if you heal someone in ETC's mosh pit, they will still be stunned, but get the 50 armor.

Basically, take whatever counters the enemy best.

Concentrated Doses: if you took Detachable Box Magazine at level 1, then you should be trying to stack doses anyway. Mine as well get more healing for doing so.

Sharpshooter: similar to Nova's Snipe master, this lets your healing dart heal more: if you can hit it. Overall this gives about the same overall effect as Concentrated Doses, just in a different way.

Contact healing: Biotic Grenade doesn't heal that much to start with. It's strength is the healing manipulation.

Take either Concentrated Doses or Sharpshooter, whichever you are more comfortable with.

Nano Infusion means that the target of Nano Boost should never die, unless they get cc'd. It can be nice, but means that your nano target better be REALLY good.

Ballistic Advantage makes Eye Of Horus a teamfight monster. If you are good with Eye, then this can be terrifying.

Somnolent Doses: If you took Detachable Box Magazine at level 1, you should already be staking 5 doses whenever possible. Unfortunately, the sleeping target will often be woken up quickly. Still, a second or so CC can be nice. Just not nice enough for a level 20 talent.

Dynamic Shooting lets you start stacking quickly. Try and get 5 stacks on 1 target then change targets, as Ana's damage per shot is a massive 70.
Ok this talent sucks. You are a SUPPORT, you really don't need to do damage, keep that in mind.

Custom Optics gives you 10.5 attack range, almost the same as Sgt. Hammer in siege mode. That is really nice for staying safe, but healing dart might not be able to reach your tanks then.

Overall, try everything. Depending on you playstyle, you might find that you like a different talent then I suggest here.

This is my first guide, so please go easy on me for poor formatting, spelling mistakes etc...

Comments are welcome, thank you.