I always particularly enjoyed D3's CTF mode, since it could get ridiculously fluid and fast-paced with two well-balanced teams. The sadly-underplayed Assault mode was a lot of fun, too, as were one or two of the user-created gametype mods floating around (yay Instareap!)
One of the best things about D3's CTF was that it had a 4 team variant, and that mode actually used to be played often in the Quadsomniac map back then. That added an extra strategic element to it, since you could rapidly increase your score by collecting multiple flags at once.
My favorite game type was Monsterball. There were some balancing issues with it, but it was something truly unique that I haven't seen anything like in any other game.
As for Assault, that is the mode from UT.

You probably mean Entropy.
All this talk of Descent makes me want to get the HLP multiplayer sessions going again at some point. I was actually watching several demos of our old D2 games just last month. I probably won't have time for it until the summer though.