"Best" is so difficult to really say.
Cracker factory is a great value, but IMO it's a horrible building. Just a big, boring cube.
Otto is really fun, and reasonably priced. I have him and don't regret it a bit.
Frink is good, building is great, but no questline. But I have him and don't regret it.
Jasper is a good character and the community center is a nice building, but no one uses it.
Barney comes with a great building that a lot of characters can use, but he's the most expensive character in the game.
Duffman has a lot of outdoor tasks, but most of them are the same task in front of different buildings. The brewery looks good, but takes up a lot of space, and it's also an expensive combo.
Disco Stu comes with a nice building of a nice size (that boosts consumerism, if my current info is correct) and I've seen some good outdoor tasks in neighboring towns. Also an expensive combo.
That's just off the top of my head. Were you wondering about anything in particular? I've done a lot of research on premiums so I don't waste donuts. (I don't have a lot of donuts to spare.)