Each of those games have their different appeals / pros / cons
All of em except, Unreal Tournament (not neceserilly its mods) have the chance to allow people to work as a team, which, for me, is the real appeal of online gaming.
Counter-strike is phenomenonally popular. due to strong teamplay and teh amount of tactics that can be used.
Warcraft III is great too, tho I'm not really stong at online RTS games..
Diablo II has an enormous online world and the ability to build up your character is popular, and thenumber of unique items keeps things fresh...
Unreal Tournament is more for your fast frnatic online gaming, which is always nice...
All of em, except warcraft III, can also be played comfortably using a 56k modem too (haven't tried wc3 wih my modem)