What!? No one mentions the awesome, awe-inspiring QUAGGOTH!? They are, simply put, one of the coolest D&D monsters out there. Plus, the mini looks cool, too.
Also, DERRO. Derro, especially when mounted upon giant spiders, are friggin' cool. Half human, half dwarf hybrids that are insane and corrupted with evil magic!? How can you not love it, I say!
I also really like using the Kuo-Toa, simply because of their aquatic connection, and because of the major "underdark" races (mind flayers, drow, duergar, etc...), they are the only one that is "fading" - they are past their zenith as written in the rules, and I like that... very lovecraftian, I think.