Praevus alongside the Heresiarch are the best bosses of the HeXen series. Eidolon, Korax and D'Sparil are a disappointment, just a "shoot at it until it dies"-like boss.
Regarding HeXen 2, it's a good game, although scaled down from HeXen due to the limitations of using a 3D engine back in the day, so we've got fewer enemies and smaller stages, that you can take it as a positive thing since the puzzles aren't too spread along the hub. The new classes (Assassin, Demoness) are fun to play. The only negative side to HeXen 2 is the setting, making Eidolon into a demon and using the four horsemen of the apocalypse is not very creative. The hubs also aren't innovative: Medieval, Aztec, Egyptian, Greco-Roman and Asian in the expansion. But perhaps that's my retroactive experience playing so many games with such themes so I grew tired of exploring the same aesthetic.