Definitely Toad is the cutest. Sackboy isn't really that cute, kinda creepy actually.
The better question is which game is prettier. I thought I would say LBP3, but watching the videos TT is very pretty with an old school polished look that I really like. And everything seems to be animated in detail from Thwamps to Toad moving his arms while walking under water. I think I like the look of TT better...