Favorite is always going to be different for everyone, but for me it's gotta be ASSASSIN ROGUE
Sure everyone has a reason why other subclasses can be optimal for one situation or another, but for me, Assassin is still the most fun personally.
You get a huge payoff if you manage to sneak up on your opponents and can do a disgusting amount of damage if you do it right. Certainly it's not for everybody but I've been able to solo some act 1 bosses in a single turn just by carefully planning my ambushes ahead of time. It's a seriously front loaded build, meaning that if you do it right you will make any followup combat trivial, assuming they survive your alpha strike.
Also this subclass pairs ridiculously well with Gloomstalker Ranger, more attacks, more initiative, more of that turn 1 beatdown. However I wouldn't recommend doing anything more then 5 levels of Ranger, that just cuts off too many sneak attack dice, plus you don't want to miss out on evasion!
p.s. not sure about the others but I should note that I've not had much trouble getting surprise on my enemies, I do try to sneak up on them so that no one in my party is seen until I initiate combat from stealth so that probably helps.
