Well, I have them all.

My favorites are Moleman, Frink and Otto – but I also like the Squeaky-voiced Teen, and Barney.

It’s best to buy one that comes with their own building and for that you really can’t go past Otto or Frink. For many of the others, you need to be quite well-progressed in the game for them to have much to do (and if you’re on a budget and didn’t get Barney at the Christmas update, he’s great but the most expensive of the lot).