Sylvanas is both a specialist and a ranged assassin. She uses 2 talents on levels 1 and 7 for pushing lanes and all other talents to boost her damage. This makes her a strong pick on teams with few assassins(double tank/supp teams).

I pick Sylvanas mostly as a counter pick, 2nd or last pick, against teams with few nukes(nova, li ming, chromie, zera...).

Before her rework I used her as a safe pick, playing the Paralysis dagger build to deal damage from afar if countered, but on current patch it feels a little lacking in damage so I only have it as reference.

She is also a very valuable pick on Towers of doom, she can easily push 2 lanes+ get mercenaries on her own and break tower walls from afar for an early advantage.

After level 7 using a black dagger and spamming withering fire can melt minion waves while staying above 3-4 stacks.

On level 10 choosing an ultimate is completely situational. If the enemy team plays defensively and groups up for teamfights or they have a valuable healer like morales Wailing arrow is the way to go. But, if the enemies usually spread out and you can find isolated targets mind control can melt them.

The rest of her plays consist of going near squishy targets and unload withering fire and auto attacks on them, using haunting wave to chase them and refresh withering fire charges.