Ben liked turning into Whampire and Chromastrone the most. No one quite understood why.
Of course he turned into other aliens when the situation called for it but if he had the choice Ben would always choose either Whampire or big chill. This is because of one simple thing, turning into one didn't hurt as much as everything else.
No one ever thought of how Ben felt having his body and biology constantly change and by the time Ben could probably communicate it he learned to simply suffer in silence.
The beginning of Ben's years were hell, every alien he had deformed him in a way that was extremely painful, so painful it almost traumatized 10 year old Ben. Heatblast made him feel like he was burning alive, Cannonbolt gave him several concussions, Grey Matter made it feel like someone else was controlling him, etc etc. However one Alien didn't hurt to turn into, Ghostfreak. He didn't quite understand why it didn't hurt to turn into Ghostfreak or why all that felt different was being cold but he was grateful. However when Ghostfreak broke free from the omnitrix Ben couldn't turn into him anymore, so he had to suffer in silence while his bones and biology constantly contorted.
After a few years Chromostone became a similar scenario, while he had to deal with the pain of having his muscles and bones change, Chromastone was relatively human shaped so it was bearable at the very least. However what made Chromastone bearable was his unique biology. Crystalsapian's run on pure instinct and because their practically living rocks they don't feel pain or sickness. So he doesn't have to deal with any human defying biology, just emptiness.
Big Chill was preferable as well, he was similar enough to a human's biology that it didn't cause excruciating pain (at least now) and he got to fly, which Ben would gladly take in exchange for the constant frostbite. However after the incident in which he was impregnated by Big Chill he could no longer stomach it, he never willingly transformed into Big Chill again, and if he did he was extremely distressed after the fight was over.
Whampire is probably Ben's favorite Alien to turn into, during the night at least. It's almost human shaped which helps alleviate the pain (now to a level almost tolerable) and the biology felt remarkably similar to humans. The only difference felt was a minor chill, which he would gladly put up with in exchange for feeling relatively normal.
Being a hero hurts Ben, but at least these two aliens make sure it doesn't hurt that much.