This one is hard for me, I have a favorite five but not in any particular order:
1.) Merrill from DA2 - the semi-Irish accent matched with a bit of light hearted childishness all wrapped up in a decent mage makes for a fun character. I'm still working through DA2 but I already like her a bit more than Isabela (although Isabela's outfit... grrrrrlllll)
2.) Korgan from Baldur's Gate 2 and ToB - Ok, I have to say that violence is funny, violent people are funny and dwarves are funny; put it all together and you get Korgan. He's my insurance that I don't romance that little crybaby Aerie, in addition he's a decent fighter and he puts everyone in the party on edge. There's even a dialogue where he gets into a rage and attacks you and your party, how crazy is that! Yes, he's chaotic evil, but I'd have him no other way.
3.) Sarovok from Baldur's Gate 1, 2, and ToB - I have to admit that I actually liked Sarovok better than Irenicus, don't get me wrong, both villains were awesome. However to me Sarovok was less emotional and even more calculating than Irenicus, think about it: he kills your foster father, sabotages one mine to take over another (filled with slaves), has all of the inhabitants of your childhood home killed and replaced by shapeshifters, killed his own father, business leaders, and political rivals, all in the goal of starting a war just to get an initial slaughter to become a god. Plus, in ToB he makes for an awesome ally!
4.) Liliana from DA:O - A redhead French spy (ok, Orlesian bard, shut up), what's not to like. Sure, she's pious and sometimes gets holy, however she does a good job of not getting preachy. She is the equivalent of the Christian girl who doesn't judge because she's done more naughty stuff than you have (and she's French!).
5.) Canderous Ordo/Mandalore from KOTOR 1 and 2 - I've always had an admiration of the Mandalorians, they seem like the Spartan culture in the Star Wars universe. Ordo shows that even after the big wars are over, in fantasy it just means that you have to jump into the little wars. Plus the Mandalore armor looks awesome and who can deny a guy with a huge gun.