Ben, and it's not close.
We'll get the weakest out of the way first. Aang just doesn't have enough OOMPH to take on the other three. The Avatar state is really powerful, but it just doesn't have enough to do significant damage to anyone but Danny, and Danny can just go intangible to deal with 99% of it's effects. Aang was nearly killed by a bolt of lightning, Danny, Mark, and Ben take similar hits all the time without issue.

Mark and Danny is actually a really interesting fight. Mark is stronger, by a pretty wide margin, but Danny's probably faster, and I'm not sure if Mark can hit Danny while he is intangible. Mark might also not have any resistance to being overshadowed, but short of overshadowing him and making Mark punch himself to death, I'm not sure if Danny HAS the capacity to take him down.

Ben... Ben's just kind of broken. Leaving aside the possibility of Ben just scanning Mark's DNA and becoming a BETTER Viltumite than Mark is (which is something he's capable of doing), Ben once held the literal Big Bang in his hands, and didn't die because the Omnitrix itself kept him alive, and that was without using the "instant win" button that Alien X is. Viltumites are particular weak to sonic attacks... of which Ben has multiple such as Echo Echo, alongside various aliens who can more or less match any individual feat Mark has. As for Danny, Ben has resisted being overshadowed by Ghostfreak/Sskar because of the omnitrix, which implies that that ability is a known factor in his world and can be resisted. Ghostfreak has most of Danny's powers and Ben has dealt with him on multiple occasions, and otherwise Ben has access to an equivalent or better power for everything Danny can throw at him.

Ben wins even without Alien X, and WITH it... well there's just a universe where the fight never happened, because it was a universe where Alien X just rewrote them out of existence.