(English is not my mother language, bare with my grammar mistakes.)

Sylvanas is one of best hero in Heroes of the Storm. Her trait makes her push power overwhelming. I highly suggest her in maps with Bosses and Boss like map objectives(e.g. Tomb of the Spider Queen, Battlefield Of Eternity , Infernal Shrines, Garden of Terror etc.) After getting boss, if you push with boss whole team together with Sylvanas, you can go to core so easily.

Besides of pushing power, she is also a good team fight companion. She can deal damage to all enemy team with Shadow Dagger and Wailing Arrow.

Keep in mind, Sylvanas is a fragile Hero. If you get stunned and focused, you can die in a second. I don't suggest solo push so hard. It's tempting but if you can't see 2 or 3 enemies on mini map, just fall back and run for your life.

Heroes of the Storm a team fight game and losing one hero on just before objective or at late game can cost you game. If you want to push. You call team mates and push together. Sylvanas player have to stop towers from distance and suddenly under tower fight turn into open area fight.

19.07.2016 Update: I changed Mind Control ultimate to Wailing Arrow, as far as I see people die while using mind control recklessly. If you are not feeling comfortable with 3 second stun to yourself, don't pick Mind Control.

	Overflowing Quiver: Pick for extra damage to minions and mercenaries and quick wave clear. I don't recommend. Since it doesn't damage to heroes, useless in team fight and during team push to enemy forts.
	Paralysis: Increasing duration of Black Arrows allow you to stun minion waves, mercenary camps and towers. You can get siege camps without getting damage. And this makes easier to get Bruiser Camps. Most importantly, if you are pushing towers and forts, this ability gives you chance to focus enemy heroes after stopping forts. You can protect your boss more easily
	Mercenary Queen: I never recommend this. Paralysis is way better then this.

	Withering Barrage: You can get this if you are going for Windrunner. You can deal burst damage to an enemy hero with full of Withering Fire stack. Theoretically with Windrunner you can hit to a enemy hero 18 times with your Withering Fire.
	With the Wind: Increasing range might be useful to hit a running enemy. If you are running away from enemy gang this talent useful with Evasive Fire talent. I personally play with team and never choose these two for team sake.
	Lost Soul: Personally recommend this. This is very good talent for team fight. W build is a late game investment but it gives opportunity to push harder and deal damage to whole enemy team, towers and minions. If you are pushing a fort and enemy team try to defend as whole team, it's a lucky day for you. Even 1 hero gives you 1.25 seconds cooldown. If all enemy heroes there, total 6,25 second cooldown on Shadow Dagger is insane value. After completing your Shadow Dagger with Life Drain and Cold Embrace, you can deal insane damage to enemy heroes with your Shadow Dagger and get heal from it. 5 vs 5 team fight you will get heal and make enemy vulnerable every 6 seconds. Better then Tyrande's Hunter's Mark. And you deal damage to all enemy team if they sticks together.

	Barbed Shot: This deals extra %200 damage to minions and mercenaries insanely. Quick wave clear. Barbed Shot easily clears minion waves and gives opportunity to take camps more quickly
	Unstable Poison: This talent is good for wave clear but does not provide extra damage on mercenary camps.
	Possession: I never recommend this. It look like gives you power to hard push but Barbed Shot is way better then this. Barbed Shot easily clears minion wave and gives opportunity to take camps more quickly

For ultimate power, you need to decide which one is good for your team composition and enemy team composition.

	Wailing Arrow: This talent has huge value in team fight. Especially good versus mages and healers. You can stop channeling abilities with silence(e.g. Li Li and E.T.C.'s ultimate power ) You can use this before team fight and stop enemy team to initiate a fight. While silencing them, you also make a good amount of damage.
	Mind Control: This talent is good for getting a weak enemy member to near your team and killing one of them before fight start. You can stop Illidan to abuse your team. You can bring some heroes like Lt. Morales or Li-Ming to front line to kill easily. These type of heroes generally fight at back and reaching them is so hard.

	Life Drain: This gives you sustain. When pushing a fort with team, you use your Shadow Dagger to stop all minion wave and towers. Each hope of spread gives you heal and which means if you play carefully you will not die easily. Personal choice of mine.
	Windrunner: This gives insane value to Withering Fire you can instantly burst a hero with maximum 18 Withering Fire. This talent requires skillful game play. Need practice to use your E and Q quickly.
	Will of the Forsaken: I never recommend it. You are a fragile hero and you should fight at back of your team. You should not be in front of 5 enemy heroes. If enemy focus on you even you are at back line, you can easily run with your E power. while your teammates deal with diving enemy heroes.
	Spell Shield: If you are getting heavy spell damage from enemy, you may pick this. But other two options( Life Drain, Windrunner) are very good to pass.

	Evasive Fire: This gives you good fight chance to able to handle 1 vs 1 fight. If enemy running you can chase them easily and if you are in bad condition you can run away speedily. However my build or more focus on being with team so I don't recommend
	Cold Embrace: Making an enemy vulnerable for 2 seconds, I am in. With getting this talent Lost Soul has so much value. In team fight Shadow Dagger build gives heal and cooldown reduction up to 6.25 seconds. This means you can make a hero or a structure vulnerable every 6 seconds.
	Overwhelming Affliction: This has good value versus a running enemy. In team fight if you want an enemy hero down, you can just slow them. When this combined with Paralysis and Lost Soul this talent will easily stack up and slow enemy insanely. Easy kill for your team.
	Remorseless: Sylvanas is one of the high attack speed hero and you can spam Withering Fire, so your basic attack gets %40 bonus almost every time. With this your auto attack will surprise enemy. You become an assassin

	Deafening Blast: Silence in double time and %50 more damage. This means 5 second no ability use for enemy team. you can see enemy team running from your team after using Wailing Arrow.
	Dark Lady's Call: Getting extra speed may look like good idea. However all other choices way better then this. I don't recommend.
	Fury of the Storm:This give you opportunity clear minion waves instantly. And your basic attack versus enemy heroes and structure will be increased. This ability will give you powerful basic attack damage. My personal choice for late game. This also stack with Remorseless.
	Bolt of the Storm: This gives you mobility. You an use this vs heavy stun enemy team composition. However other abilities is better then this.

I mostly play for team and in my opinion, Heroes of the Storm is all about being team.
Sylvanas is the one of best push power hero. However, in early game she is so fragile. Be careful for pushing towers. If you can see all enemy heroes in mini map you can push towers. But if you don't, stay back and just get experience to improve your hero. Mostly save your Haunting Wave for escape. Also Haunting Wave is good to clear Zagara's Creep Tumor. You can uncloak enemy heroes with Haunting Wave.

Sylvanas can kill mages in 1 vs 1 fight but you must be careful about not getting their ability damages. You can use your Haunting Wave to get closer to them and you can kill them with your basic attacks.

In a fight versus a hero which can stun you, you should move rapidly and use your Withering Fire and Shadow Dagger. You can hold versus two heroes on a line with this way.

I rapidly use my Withering Fire like spamming. It has 2 seconds cooldown and use no mana. So it's free damage to enemy from no where.

If you are playing on a map with boss, and your team gets boss. Ask your team mates to push with boss. Sylvanas has a high pushing with boss. You can even finish game in a map like Battlefield Of Eternity in 10-13 minutes.

As a personal experience, I find The Lost Vikings and Sylvanas combo so good. While The Lost Vikings soaking lines, you and your other 3 team mates can push enemy towers and forts so hard. Enemy team will be shocked when they see they lost their all front forts, even in 7-10 minutes.

I find @Arkenge comment on slow build is very helpful about game play and adding here:
"When you take Paralysis, Lost Soul and Overwhelming Affliction, you are becoming a nightmare in team fight.
Your Shadow Dagger becomes almost always available and when it spreads, all enemy hero get the 6% slow. This and your basic attack stack very fast and, if you manage well, you can very often slowdown the all opponent team by 30%."

This is the end, If I am helpful, I would be happy.
Have fun in the nexus.