Quake 4 stinks. Full stop. I've got the demo which has single and multi built in.
The multi is kind of similar to the old Quake, but not as good. It's not as smooth or fast, there's no background music, and none of the hilarious comments like "Bulldog blew himself up", "Darkstar left a large crater" etc. None of the guns have any real power, and the rockets move too slowly to be any use - by the time they get to where you fired them, your enemy's moved on already. There's the usualy jumping around from side to side on the bouncy ramp things, but it's just not as fun. It's slower, like I say.
The single player? The only way of describing how bad that is ain't appropriate for this site... it's that bad.
As for my favourite online game? It's probably either Wolf:ET, BF1942 (best of the series IMHO), SOF2 or even C&C Generals:ZH if you played with fellow noobs, or your actual real world friends.