Starcraft rules!!!!!
As a 'Mac' user even i can comment on this thread.
As far as i'm concerned, for this genra of game, they don't get much better than starcraft. Me and a collegue at work used to play a 'melee' against each other across the LAN. It was only a demo we had but we played it for hours and hours, such a great 'Vs.' game.
I eventully got hold of a full version - damn Mac games never go 'budget' - and tried my hand at the scenario builder, but could never get them to work quite as i wanted.