If chosing between Assassin and Warrior, go Warriors hands down. Does a lot more an Assassin could do role-wise and don't bother with SF because that will get nerfed soon (how I'll love to see the sin population nearly extinct after that

).
It's a pretty balanced class, both PvP and PvE and is definatelly the best choice for Hard Mode from a few builds. It's also very versatile IMO. Assassins are none of this imo.
As for mesmers, they're kings in PvP but not really the best choice on PvE. Elementalists, meh. They're ok I guess.
Bottom line: Warrior. BUT, you kinda of actually have to know if you enjoy playing as a melee class you know. Those 3 classes play out very differently.