For me, it's Fight Night Round 3 - it was pretty much the first decent boxing game for any platform, and probably still just shades FN:R4 even now - the battles you could have with friends on that game were amazing - last-gasp knockouts, 9 rounds of beating your buddy to a pulp until he can't stand up any more, even those battles that could basically take place in a phonebooth - you both just stand there and throw punches until one of you falls down. Just a brilliant, brilliant game.
Also worth a mention is Burnout 3: Takedown. Still to this day one of the finest Arcade Racers I've ever played - it had everything - weaving in and out of traffic at hundreds of miles per hour, just missing trucks by inches, then at the last second, slamming into the side of your friend's car and sending him head-first into a concrete wall. THAT's what friendship is all about.
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