It's the best because it wasn't kill streaks focused and its maps were top notch plus the player count was smaller therefor less annoying players online, especially hardcore mode. I can go on but I'll just leave it at that.
because it was ground breaking. because the maps were great. because KSs werent a big deal so everyone did play like a puss. because it actually had things to climb. because trying to shoot a chopper down with an inaccurate rpg was cool. because QSing wasnt a big problem.
If they released a new version of COD4 (no glitches, hacks, 4 new maps) i'd buy it.
It was the first one people played. Like pretty much every other time people like the earlier things better.
In reality it's a broken mess online with ridiculous explosives, massively unbalanced perks and horrible linear maps that are only good for sniping and random explosives (and only ever played out with those dominating).
But it's the one that made CoD popular, that makes it good right?