I personally love Scarlet-Speedster most, cause Speedsters are my favorite power-type, And Wally West Flash is my absolute FAVORITE SuperHero!!!
Now that said, Hyperman and the AllMighty-X are both very close seconds, and I think are neck-and-neck in terms of raw-power. AllMighty-X has the advantage with his Reality-Warping, but Hyperman's energy-level protects him from existence-erasure.
As for Lex-X, he's part of a cool, DC/Ben10 hybrid universe I'm coming up with. He's a good-guy version of Lex Luthor who, when he was 10 years old, discovered the crashed Omnitrix while on summer-vacation to Smallville. Still figuring out the rest of the Story and Universe, but that's the basic pitch. I also have a concept-art/costume-design in my head for 10 year old Lex, and his version of the Omnitrix. Sound cool?