AOE2 is v. cool but my 56k modem wont allow online play (cutoff after 2 hours :().
CS1.4 is very cool, best version I have played but laggy on some servers.
And the King of all great games has to be Worms - in any format you care to mention. It is so great its unbelievable, I have to get a copy of Worms World Party, or whatever the online version is called!!