I've used everything. The magic-back sleeves are the best product I've come across. Despite what your previous experiences with Ultra Pro brand sleeves have been, I strongly recommend giving them a try.
I can't stand these sleeves. They feel cheap and break often. They're also very annoying to shuffle - I can't really describe the feel, it's just awful for me.
KMC 100ct. packs, however, have been nothing short of amazing for me. They're the most consistent I've used, quality-wise, and they last long enough.
If you get a good pack of Dragonshields, though, they are IMO the best sleeves period. It's too bad most of the time the sleeves are horribly miscut or have awfully frayed edges, making them pretty much unplayable.
I never split KMC 100ct. (don't get me started on 80ct. packs) or Dragonshields. I don't know what you guys are doing that makes these things split. The Magic ones split because they're just low quality - consistently, in my experience.
As for stiffest sleeves, I have yet to try The Hardest Sleeves, or whatever they are called. I think nikofromtokyo sells them..?