Zhent and owlbear were surprisingly easy IMHO (not any sort of "how cool am I?", I'm pretty crap at combat and tactics). Owlbear loot was vaguely interesting, Zhent loot was quite profitable.
Other difficult but optional fights might include the one for the sussur bark, which IMHO isn't worth it as it gives you a +1 scratchy weapon like a greatsword, but by that time I'd already amassed a collection with the same stats; the creature guarding the ruined tower seemed tougher than it was and the main bonus was being able to wander around the Underdark without the daft great lummox bashing my entire party off my feet; spider matriarch is just too moody, and while I may like spiders I don't like *her*, so I'm leaving her alone; er... that's all I can think of. There's another couple of mobs I have yet to deal with.
The critters guarding the sussur tree are a rare example of being able to not have to deal with everyone at once, so I just used my archer and buggered off when it looked like it was getting a bit hairy; and it's not necessary to deal with them when rescuing the idiot.