That's one of those questions with a million answers.
That said... I've got a 29g US peaceful community. I'm still stocking/restocking at the moment. 29 gallons is less space than you think, so the key is to take advantage of the height as much as you can. Basically, you're looking for bottom level schoolers, mid/top level schoolers, and a centerpiece fish.
What I would suggest is to pick your centerpiece fish first, then make the rest of your stocking decisions around that. What did you have in mind for a loner (or pair)?
Some possibilities for consideration:
2 Pearl Gourami (right size, right temperment compared to other gourami, though many of the smaller gourami would work)
or
1 Angel (eh. on second thought, don't. they really should be paired and have more space.)
or
2 rams or apistogrammas ("peaceful" dwarf cichlids. everyone that has them seems to love them)
or
a few livebearers
That's just a few of the possibilities. What do you really want to keep?