Okay, so 'best' is super subjective, but if we're talking emotional depth, you have to look past the obvious 'Ben x Kevin is inherently deep' take. A lot of fics just rehash canon angst—Kevin's past, the OSMOSIAN drama, yadda yadda. The real gut-punchers for me are the ones that explore the aftermath of 'Alien Force' season 2, when Kevin was merged with the Ultimatrix. Fics that treat that not as a monster-of-the-week fix, but as a lasting trauma? Those wreck me.
There's this one story, 'Circuitry and Stone,' that frames their relationship as Kevin literally teaching Ben how to touch him again without triggering a PTSD flashback. It's less about romance and more about rebuilding fundamental trust. The ship feels earned because the emotional work is the plot. Another angle I rarely see but adore is post-'Omniverse', with an older, maybe-retired Kevin. The depth comes from mundanity—Ben dealing with legal loopholes to get Kevin pardoned, Kevin trying to hold a job, the quiet burden of a shared history no one else fully understands. That 'domestic' tension carries a different, heavier weight.
Honestly, skip the high-school AUs for this pairing. The depth is in the canon-adjacent scars.