Chiming in late, but Red is of course my classic choice.
But I also like the White dragon as one getting little love, typically presented as a frost giant’s glorified guard dog. Yet to me the whole concept of a dragon whose primary existence is centered around the cold has so much adventuring potential— a shape glimpsed in a swirling blizzard, a form that emerges from the snow as if formed by it, caverns glittering with ice... brrrr! I think that perhaps people get so caught up by knowing that they are technically the weakest dragons, and forget that they’re still dragons. (Which is a great way to catch a party of jaded players with their guard down...)
“New Rule: Don’t touch the Evil Necro-Stick!”— Teen player in a recent campaign. Cue evil DM laugh.