SQ, CX, KA and any airline that ever says 'Mr. LT do you mind being upgraded?'. Actually I like those airlines that say that the best and I recommend them to all my friends.
Being upgraded is a bit like getting a BJ off a fit bird when you aren't expecting it - nice.
OK it's not exactly the same but it's pleasurable and in that sense therefore it is the same.
Looking at it another way upgrades last longer so they must be better. Some upgrades can last for like 13 hours or so - I'm not sure a 13 hour BJ would be something I could stand - I'd try though - I'm like that - one of lifes triers.