Curious you should eliminate Gaunt and his Ghosts, considering the Iron Snakes in particular hold Gaunt in high regard, and others think very highly of Mkoll, the same Mkoll who has
Don't underestimate the Ghosts, they're pretty hardcore elite. Eisenhorn and Ravenor Jane both accomplished feats that would make Astartes take a healthy amount of caution. Whilst I've only just started reading the Cain series, but from I've read so far and from little glimpses he hints at in his accounts, he's taken on some pretty hardcore opponents.
Whilst I love Astartes as much as most people. It's almost always the humans who are the most interesting, especially when the accomplish these amazing feats, perhaps because Astartes are expected to do such feats, so whilst still cool to read, it's not as surprising.
I couldn't pick a favourite fight, especially without spoiling a lot. Many of the fights in Eisenhorn, Ravenor and the Ghosts are brilliant. Lokens first fight against Lucius, possibly the shortest fight ever, but so, so memorable and epic. For all the hate and poor quality of the rest if the novel, I still enjoyed Sharrowkyn vs Lucius. Grammaticus against the Lucifer Black in Legion
, or the breif final fight between Sheed Ranko and Chayne.
Though if I really thought about it, whilst I can't pick a favourite, one of the best written and brilliant to read would be Xarl vs the Genesis Marine Champion. That was just awesome beyond words. Although spoiler regarding the fight: