I like guppies (and platties) for a lot of reasons. They're easy to breed, are not scared of you, are well adjusted to tanks, and there's always cool new strains coming out that look amazing. It's fun to work on your own but to do it properly you need about 9 tanks. I read a good article on the net a while back.
EDIT: Yes Dan is right, I looked into a while ago as there are top discus breeders in Australia who were willing to ship some to me. You have to have them quarantined at a MAF facility for 6 weeks and if they die during that time it's your hard luck. Considering I have no idea if they'd ever do a water change or use filters, I decided not to risk it, especially with discus coming from environments that had daily water changes. It's a lot easier if you can find a local breeder or get your LFS to get them in.