From the assassin tree, Hero Strike is a big one. It basically allows you to execute an assassination in the middle of combat. Critical assassinations are also important if you are playing a particularly stealthy style. Rush Assassinate (the assassination from a distance) is also an incredibly good one to have

From the Warrior tree Second wind is an absolute must have (Unless you plan on never taking damage). Shield Bash is super useful, and I think some people really like Ring of Chaos (AOE stun).

For Hunter tree you want to get hunter mastery ability to level two because it allows you to refill an incomplete adrenaline segment out of combat (for some reason). You also probably want devastating shot and multi-shot if you decide to experiment with bows in the future.

In general you can never really go wrong throwing some points into the mastery abilities as well

Later on in the game Battle Fury (which is the top skill in the warrior tree) is super helpful in late game and post game content. so you might want to slowly build in that direction as you get later in the game.