[Benn Roe]

I think "classic" is most useful as a way to describe a character's definitive look, the one they keep coming back to, but for a lot of people it's just the look the character had when they first became aware of them, or even just their first appearance. Here's a classic X-Men wave that pulls from each of those definitions.

Angel: definitive outfit with halo on the chest
Banshee: definitive first-appearance/Giant-Size/etc. look
Havok: definitive first-appearance/Outback/etc. look
Longshot: definitive first-appearance/Outback/etc. look
Magik: New Mutants look
Sabretooth: first-appearance/Marauders/etc. look
Shadowcat: Excalibur look
BAF: Blob: definitive jumpsuit with yellow stripe for a belt