I don't really have one favorite, but I like any of the ones that involve collecting things (physician, wizard, etc). Plus, I like giving them the drunkard flaw, so all that bloodmoss comes in handy.
As far as least favorite, I'll add my vote to those who dislike the merchant. I find all the trading and stocking and price-checking and schmoozing to be little more than tedious busy work. I try to avoid quests that involve the merchant.
I also learned my lesson about not letting the game premake a hero for you with my first merchant--who wound up with the misanthrope trait. I got so tired of constantly cancelling his actions just as he was about to call someone a lack-witted cur that I wound up sending him to the pit. And I had to do it twice, because the first time he just crawled right back out!