You can argue about Wolverine and Cyclops all you want (which is a stupid debate because obviously Gambit is better), but the best one-time throw-away X-Men of all time is obviously Morph.
You don't know who the hell Morph is, you say?
Morph in the X-Men animated series
Morph (voiced by Ron Rubin) was cast for the X-Men animated series as a "throwaway" character for the writers to kill in the opening story arc to sell the stark nature of the series. Changeling (Morph) was chosen because of his past ties to the X-Men, as well as his long absence from Marvel Comics. Being a deceased minor character in Marvel canon, he was a "blank slate" and the writers were free to do with him as they wanted without fear of reprisal from fans of the character. Morph's death was intended to be permanent, but he gained unexpected popularity, and was brought back.
Sydney's codename had to be changed from Changeling to Morph as DC Comics had trademarked the Changeling name for the character who later became Beast Boy of Teen Titans fame.
Still don't remember him? Well he was the guy who could (get this) morph in to any other character. So pick your favorite X-Man... yup, Morph could be him or her. With the added benefit of being anyone else, too. Oh, he's your favorite X-Man and more?!? There's no conceivable way to beat that.
Fuck off, Dunn and Meg.