In the book version, I would definitely go for Stannis. He has the best claim legally, and even though he’s a hardass, he is the one who stands up and does the right thing for the realm (without getting too specific for spoilers’ sake). He has a lot of flaws, so he isn’t an ideal king, but none of the choices are. Dany has strong potential as a ruler, but she is still learning. She may be the better choice after ADwD. Tough call.
On the show, Renly is a much better potential king than he is in the books. His legal claim is still crap, but he does want fairness, he’s intelligent, and would probably be a good ruler with the cunning of the Tyrells, their sheer numbers, and their money supporting him as well. If I were living in Westeros, I’d have been more inclined to throw my lot in with Renly and his less scary style.