Harmony, I swear I am not going out of my way to disagree with you, but I always thought Jack Lemmon, while solid, played more of a schtick in many of his roles than a character. Mr. Roberts, The Apartment, The Odd Couple, it seems like his delivery, his mannerisms, they are all the same. That isn't a knock on his talent, it is unquestionably there. Hey was just one of those actors where I see him on the screen and think "Hey, there's Jack Lemmon." But then there is Glengarry Glen Ross, where he puts on an acting clinic (with the rest of the cast).
Can't argue with Gene Kelly. Only Singing in the Rain stands out in my memory but damn what a landmark performance.