Hmm. The most average as in most mediocre, or as in the most generic? Well, I'll answer both. I can only think of 360 games, since I've probably forgotten far better contenders for this title from the PS2 or PS era. But, then, if I could remember them after that long, they wouldn't exactly be average, would they?
For most mediocre, I'll say Dynasty Warriors 5: Empires. I think that's right. Might have been 6, but who cares? It's pretty generic too, but, I mean...a game has never provoked less of a reaction from me. I've liked other Dynasty Warriors titles as a guilty pleasure, but Empires was just...functional. There was literally no substance to it. So much so that I can't call it bad. There was enough variation in characters to keep me playing longer than I should have, but I don't remember a single distinctive thing about it.
For most generic, as in the game that is the most like everything else, or the least distinguishable from any other game, there's more than one contender. For simplicity's sake, I'll choose Ninety Nine Nights, though. N3 was a God Of War ripoff set in a Lord Of The Rings world filled with Final Fantasy XII characters and it played basically like Dynasty Warriors with a hint of Onimusha.
Mind you, I'm not including tie ins to movies or licensed games in this, because, if I did, I'd be here forever, simply trying to remember anything at all worth remembering about them. And anything that provoked even the slightest reaction from me, or had even the slightest bit of redeeming fun value when played with someone else is disqualified for being memorable.