Wheel sizing, backspace, ride height, and vehicle assembly tolerances all conspire to detemine what will fit or not. We have a member on here who has 275 on the front on a 17" rim with the proper backspace. I've got the equivilent of a 255 on the front of mine with a 15" rim. My set up will clear all the frame and suspension components, but could contact the fender brace under extreme compression, and my tire is slightly under 26". However, because of the suspension set up I have, it isn't going to go that far unless somethnig drastic happened, like doing a Dukes style jump. A taller tire would only have more contact potential.
As far as the stickiest 15" tire, you have to look at what is easily available and reasonably priced to break those out. As such, it may be the MT SR or the Maxxis Maruader. Both are H speed rated. The Maxxis is under 300 UTQC and the MT does not have a UTQC rating. MT is claiming 15-18k miles are possible on it, which means it is a pretty soft compound. There are softer compound 15" tires out there, but not all are d.o.t. approved, easy to find, nor cheap. However, if your willing to pay and wait, there is a company out of England that makes a d.o.t. approved, VR50 series, 15" tire, that is as sticky as a racing slick, for the low cost of $280-300 each.