Played Hellgate its fun at first, but got old incredibly fast as stated above very repetitive scenery, and too many pasive skills. The one thing I really liked was the graphics, but everything else was way below par.
As for Mythos I haven't gotten a chance to play it, but it looks too cartoony for me and not as dark and serious as I like.
And last, but certainly not least Diablo. I believe this is the best out of the three games. Amazing storyline and plot, the gameplay is very addicting, and there are so many ways to build characters. The downside is of course the graphics are below par for the time period. Resolution could have been higher, and they could have been better overall.
1) Diablo 2
2) Hellgate: London