My own take on it is that every cup game is a meaningful game in that if you lose you're out, if you win you're through. So pick 5 games moyes has won that matters, that's any 5 cup games he's won. Easy enough done.
League games matter but you have leeway, you can lose a single game and still comfortably win the league.
Big games, to me, are club defining games and since the glory days we've had coventry and wimbledon at home (club defining in the worst way) and the three finals (liverpool, man u and chelsea) which are club defining in a better way .
Past those five, the only ones that come close to being real big games as in winner takes all, win and this happens, lose and this happens are the semi final last year (because of the chance to cement a local power shift) and the villarreal play off (because of the CL money).
All the other 'big' games people mention (the game at anfield in 2007 that put us out of the hunt for 4th, the wigan and reading cup games, the semi vs chelsea in the league cup, fiorentina), they weren't big games, imo. They were important games (as were the equilivant games we've won, the quarters vs west ham, boro and sunderland, the league game vs man u in 2005 etc) but even if we'd won there still was a lot more to do afterwards.