Season 1 - Clearly the winner, Kelly Clarkson, is the most famous idol from that season, perhaps the most famous overall.
Season 2 - Runner-up Clay Aiken got a lot of attention over time, perhaps even a bit more than the winner, Ruben Studdard, however, at this point, it might be #3 Kimberley Locke who ends up most famous of the three.
Season 3 - The winner, Fantasia, has been significant more famous than the runner-up, Diana Degarmo. In that case, however, #7 Jennifer Hudson is perhaps the most famous of all.
Season 4 - Carrie Underwood, the winner, was clearly the most famous idol from this season.
Season 5 - Taylor Hicks, the winner, overall, has been more famous than the runner-up, Katherine McPhee (but I couldn't explain to you why). Regardless, #4, Chris Daughtry was the most famous to come out of season 5, and perhaps will rival Kelly Clarkson for most famous overall, in time.
Season 6 - Winner Jordan Sparks has been more famous than runner-up Blake Lewis. Season 6 was notoriously low-powered, though.
Season 7 - It's too soon to tell, but David Cook, the winner, is likely to have far more star power in the long run than David Archuleta.
Season 8 - While Winner Kris Allen seems pretty plain, he's got some talent and might surprise us. Adam Lambert, the runner-up, surely commands more attention, but it is questionable whether he can really do so for an extended period of time. If he does come out of this as the most famous idol from the season, it'll only be because Season 8 was sort of like Season 6 in that there wasn't as much talent present as in other seasons.
Have you seen McPhee lately? She cut her hair off completely and died it blonde. She doesn't even look like herself anymore! DeGarmo has gone the reality TV route it seems. I have to disagree on Season 8. I thought there was a ton of talent there. I don't think of it as low powered in anyway. In fact, I think it was their best ever.
Well, yeah some runner ups do better after Idol. Totally agree with Bicker. ANd plus, they're the top for a reason. I guess some fans discontinue following their favorite after a certain album comes out they start to dislike it...
Occasionally some of the runner up's can make a nice career for themselves, but it's still usually each rounds winner that gets most of the attention and success. But again, pretty much anything can happen.