I've played with almost every perk combination. Here are the ones that gave best results.
Sorry for mistakes, eng. is not my native language.

Valla is best for maps: Battlefield of Eternity, Cursed Hollow, Towers of Doom, Sky Temple, Dragon Shire. Thanks to her Rain of Vengence skill, she can change the tide of a battle. Always take Rain ult on those maps.
She is little less usefull on other maps, but still can pack a punch with first perks set.

In this guide you have 2 perks sets for a reason. One called Pure sustain, is good for learning how to play with her, because it's life regen will help you survive longer.
Other one is when you git gud with that hero. Ofc both will do a trick in HL games.

When it comes to Q and W skills you need to remember this.
Q is USELESS when you shoot it at enemy hero close to mobs,buildings because arrow will hit them instead of your target.
W is great for the starting team fights. With slow+executioner perk you will deal 250dmg per hit.

Early game:
Get yourself a lane with enemy melle hero. It will be easier to push and make enemy turrets waste their ammo.
Fighting against ranged one, will be waste of time, because both of you will be killing mobs before they get close to towers.
Try to hit enemy as often as possible, but keep a distance from his buildings.
Your W is perfect for harrasing and pushing, just don't use it all the time because it will eat all of your mana.

About mana, Valla is very depending on it and burns it very quickly. When you're out of mana often take Hot Pursuit instead of Searing perk.

Mid game:
Here we get in team fights more often, so you need to be carefull and don't rush with her.
Remember to take good position, away from heroes that can easily jump close to you(Like Tracer, ETC)
Try to not attack the tanks, but some closer enemy dmg dealers. Never, ever jump in front of the battle with your Evade skill.
You use it only when you're too close to focusing you enemy, or when some skill shot is coming at you.
Best to play with her is when you come from side, not from the way where your team is coming, but from a side.
Then attack with your W and weaken most important enemy player. And when they will be running at you, use your Rain of Vengence skill on them.
REMEMBER your Rain of Vengence skill is a game changer, don't just throw it at the start. Make sure you will
hit most important heroes with it.

When no one is fighting, try as much as possible to convice your team on capturing the camps.

Late game:
You need to be very carefull here, no pointless risking and engaging. Try as much as possible to capture mercs, but BE VERY CAREFULL with BOSS merc.
Do it only when enemy have max 3 alive players. Otherwise you might lose(Lost many games this way).
Main priority are map objectives, you do them, you win.

Hope this helps a bit, and some of you will stop doing ******ed builds with Valla, which are giving you nothing.