Penn vs Jose Aldo
I don't think any of the current lightweights have a chance against BJ. Maynard is a tough dude who is big and strong and can wrestle, but he can't compete with BJ. Maynard, however, should get the next shot at the title. Frankie Edgar will be a champ sometime in the post-Penn era, but he's not ready yet.
I would be intrigued by a Jose Aldo- BJ Penn fight. Aldo is a tall, pretty put-together 145 pounder. His height makes the jump in weight a possibility and everyone knows he will inevitably be a 155-er anyway. Plus Muay Thai fighters are nasty and I think the nastiness negates size differentials. I think that fight could be hyped well and if Aldo can pack it on weight-wise, it would be interesting. I think BJ would handle him, but it could be interesting. Besides, no one in LW can touch him anyway, so why not look elsewhere?
Incidently, BJ is pretty nasty himself. Look at the last 4lightweights he's fought and what he's done to their faces: Stevenson, Sherk, Florian and now Sanchez. These guys have lost a ton of blood and makes you wonder how they don't pass out! BJ knows how to open guys up!