R-Truth is the best candidate for the WWE Title for a lot of reasons, but I think his potential is the biggest one. There are so many ways WWE can go with R-Truth walking away as WWE Champion, and they're all very good choices to have at Mania.
R-Truth vs. Dolph Ziggler
If WWE decides to put Dolph back in the main-event scene, I could see Ziggler continually pointing out Truths' age and how it's his time and not R-Truths. Truth could call Dolph a child and point out how young he is and childish—just to name a few possibilities.
Honestly though, I don't see WWE thinking Ziggler is ready to main-event Mania.
The Miz vs. R-Truth
This feud practically writes itself. In fact, it has written itself.
Miz could claim Truth is in the spotlight because of him, and him alone and if not for Awesome Truth, he'd be nothing. Truth could counter that he got there on his own and also poke fun at Miz on several levels. (Looks, lack of friends, losing streak, etc.)
R-Truth vs. CM Punk
This is one I actually think would be better than CM Punk and Chris Jericho in terms of promos. CM Punk could point out that while both of them are unconventional, only one of them is "The Best in the World."
Truth could twist his words around and make it seem like that's his title. The fans could slowly get behind Truth until Mania, where Truth retains and CM Punk can finally be a heel again. (I think he works best as a heel.)
Kofi Kingston vs. R-Truth
R-Truth vs. Kofi Kingston is sure to be a high-octane match. It would be very fast-paced and exciting, and I think Truth could do most of the legwork in the promos leading up to it. This could sort of be like Kofis' moment.
That would be something Truth could stress, similar to how Cena stressed Punk had to win at SummerSlam to make himself legit.
R-Truth vs. Chris Jericho
Another good promo feud here, and again, we can go the same route as Punk and Truth with the twisting of the words.
However, we could also have Jericho constantly illustrate instances of R-Truth failing and himself succeeding. He could underestimate R-Truth as an opponent—R-Truth would become downtrodden before overcoming the odds and beating Jericho at Mania to solidify his career.
R-Truth vs. Sheamus
Or lets say Sheamus decides to go to RAW and take on Truth. I could see this as similar to a more well-rounded Morrison/Truth feud.
Friends becoming rivals and such, with Sheamus constantly downplaying R-Truth as a competitor (unknowingly), and with R-Truth slowly turning heel as the weeks go by until he finishes his turn at Mania by cheating to keep his title.
I think all of these are good programs, and there's a lot more they can do. WWE just has to be creative in it's approach.