I get bored with Melee very easily since the only way to have fun on it is to play with friends and I tend to annihilate them. With Brawl, I'm still better, but I don't necessarily annihilate them, so we tend to play it a lot longer.
Freshman year of college, we played SSB64 almost nonstop (I mean, literally, the moment someone walked into the common room the game was turned on and everyone dropped what they were doing to join). Sophomore year, we moved on to Melee and we played about as much at first before dying down to about forty minute sessions. Now that we have Brawl, we're playing nonstop again, but it may die down again, who knows.
So, in terms of longevity of playing time, I'd say SSB64 was actually the best. In terms of well-crafted gaming, however, Brawl takes the cake. We all love Brawl.