Ok this is basically an Ancient Spear build which after many games with Sonya I feel is possibly the best for her at the moment.

I have tried to make SS builds work as hard as this one but in quick match without a heavy CC team backing you up you just cannot stick to people and therefore cannot put out as much damage.

Sonya is 2 things at the moment - a sustained damage bruiser and a good farmer. She doesn't nuke people or disrupt or tank or much of anything else.
What she can do with this build in particular is latch onto someone and pressure them constantly until they die, you die, or they escape. The last of which is not easy if your skill shot aim is good.

Sonya can 1v1 a majority of heroes in the game by turn 10. Many people do not realise this and stick around until its too late to escape.

Standard practice to engage is to pop Wrath - Spear onto your target. Even if you can ride up to them the stun, fury and talented damage is usually far more desirable than saving the first spear cooldown.
Next depends on the hero, if they have an interrupt of any type you want to SS spamm to pressure them into using it.
If they refuse to use it and you are getting low then Whirlwind. Now they use it, fine, go back to spamming SS and your Spear will either be back up or back up any second.
If they dont have an interrupt or you know they used it already then you can WW whenever you think is appropriate.
This simple playstyle will kill most hero's 1v1. Many kills will come down to your aim with Spear. If you can consistently land this on fleeing enemies then you will be strong player.

Things to consider with this build.

Disengaging and reengaging is a common theme on Sonya.
Using Spear to leave a 1v1 into a minion wave a short distance away then whirlwinding up a significant health boost can allow you to reengage less than 10 seconds later, even if they chased you and were still hitting you. This catches many people off guard and is a very common move to play.

Disengaging to break Line of Sight with a bush or wall etc simply to wait for Spear cooldown. While not assassin burst - Spear > SS > SS is no small amount of damage. You are often in a position where just face tanking will lose you the fight by a small degree. These small interrupts gained by just walking away for a second or two swing the damage back in your favour especially against melee like Illidan or Zeratul.

Get into the idea of going in and out a lot. If you initiate on someone and they escape or an ally comes to their aid, sit in the bush and wait, goto the nearest minion and heal etc then go back in. If they stuck around many a time you will finish them off.
In out, In out, In out Sonya is that annoying person that wont go away.

Do not forget that WW makes Sonya a superb minion killer, she can clear lanes like nobody's business and the same with merc camps. Your contribution to the game should be reflected by your lane pushing and mercs on top of your hero killing
With whirlwind in general you rarely have to B on Sonya. Just find a minion wave and/or Fountain and that's enough most of the time.

Talents

-- After playing around with tier 1's for awhile this is the best IMO. Fury doesn't decay. You can you use this any time its up and store the fury until you need it while the CD recharges. When you die and respawn use this in the spawn before you go out. Fury is a not a problem with this talent.

-- This is required or people will escape.

-- The only worthwhile talent in this tier.

-- This is what you are, sustained damage and sticking to people.

-- You could swap this out for the SS talent if you wanted, but I wouldn't. People will escape more and you will escape less.

-- Required. If you miss no problem. Oh look now you have an escape too.

-- The only real optional talent for me. Hardened Shield can be good too. Do not take Nexus Blades, Sonya's auto attack is very buggy and very slow. Your damage comes from SS, Spear and WW and Spear should be sticking you to enemies.