I actually came up with the following list of big shots from the Finals after bouncing around this topic on NBA boards last month. I'm perhaps leaving something out from the 40s or 50s.
Game 7 Daggers (there haven't been many big Game 7 moments): LeBron vs Spurs, Don Nelson high of the rim vs Lakers, Ron Artest vs Celtics.
Game 6 Series Winners - Jordan vs Utah, Kerr vs Utah, Paxson vs Suns
Game 6 Series Savers - Ray vs Spurs, Kareem vs Celtics (1974)
Game 5 Series Winner - Vinnie Johnson vs Blazers
Game 5 Series Swinger - Horry vs Pistons
Game 4 Series Swingers - Magic vs Celtics, Sam Jones vs Lakers (1969), Fisher vs Orlando
Miraculous shots that forced OT but didn't change outcome of game: Gar Heard vs Celts (1976), Jerry West vs Knicks (1970)
Best Shots from earlier playoff rounds (partial list - certainly missing some good older stuff): Sampson vs Lakers, Fisher with 0.4 left vs Spurs, Horry vs Kings, Elie vs Suns, Stockton vs Rockets, Billups 35 footer, Jordan vs Cavs, Larry Johnson 4-point play, Bird to DJ, Reggie over Jordan (or his two threes at MSG).
If you only talk finals and whittle it down a little, I'd say it probably is a battle between Jordan, Ray, Kareem and Magic for the biggest shot ever made. And Ray might win that vote - even fighting an uphill battle against the iconic names on that list (Kareem's team lost anyway, Magic was only Game 4). The stakes were at a maximum, and the shot had a high degree of difficulty. Only negative is that the Heat still had to win in OT and take Game 7 for it to be meaningful. Jordan ended it (well, almost - Stockton had a good look he had to miss first).
Someday, we'll have a true buzzer beater to win a Game 6 or 7 that'll trump it.