Ahhhh yes, the option that's usually worded as "Sure, I'll help you.....for a price".

Thanks for reminding me, I was going to count how many times that dialog option came up in my next BG playthrough ;)

If remember correctly those options usually lead to less rewards, sometimes even a reputation loss.

Less rewards AND a chiding from the NPC calling you a greedy bastard :). It's okay to expect someone to do work for free but not okay to expect to be paid for your work. Who you calling greedy exactly? ;)

Wizard Slayer/Thief: Nearly total immunity to magic at high levels if Throne of Bhaal is installed. Once you reach level 24 as thief and get access to high level abilities, you completely lose the Wizard Slayer's only weakness thanks to Use Any Item. Combined with your natural magic resistance as a Wizard Slayer (26% if you leveled to 13 for two attacks per round), it is quite easy to stack enough Magic Resistance to exceed 100%, after which only four spells I've been able to find affect you at all: Time Stop, Imprisonment (however, the version cast by mages and Elder Orbs is virtually a non threat: unlike the Demilich's souped up version, the actual spell has a Touch range and a very long casting time and thus is extremely easy to interrupt), Lower Resistance (do enemies even use this?), and the Wounding ability used by Hive Mother beholders in TOB.

Wizard Slayers in general have some interesting features. One particularly notable one is that they can (and I believe may be the only class that can) actually disrupt those automatic "cheat" contingencies/sequencers that enemy mages use (the scripted casts override other circumstances but are not immune to the percentage chance of failure debuff that being hit by a Wizard Slayer induces; what's more, the Wizard Slayer does not actually need to deal damage to apply the debuff but merely must hit, so even such effects as Stoneskin or Protection from Magical Weapons do not appear to affect this).

Kensai/Thief is pretty well loved as well, both because the Kensai abilities and the Thief's backstab are extremely powerful in combination, but also as with Wiz Slayer/Thief, you get the synergy between Use Any Item and an initial kit with severe item restrictions).

Sorcerer seems interesting, although the downside is that if you are playing the original versions (as I have; I have yet to consider it worth the money to upgrade to the EEs now that the bugs seem to have settled down) you cannot start this character in BG1 (in particular this means you can't try out the Big Metal Unit without console'ing in the golden pantaloons). Notably in addition to the foregoing, only Sorcerers can get access to the Imprisonment spell during Shadows of Amn (this can be very useful, e.g., if Greater Wolfweres bug up severely it could be one of the only ways to defeat them, though I'm not sure if that's still an issue).