MM3DW1.
But honestly, I don't think we need a Hub for ONLY duel maps. Duel players would probably stick with the map with more options.
You say that, but duelists like me don't like playing on MM7SPR every one in five maps. Or MM5GYR, or other joke maps. Given the rarity of server hosts editing map rotations at all, I find this a significant contribution to the duel scene. (That is, of course, wouldit catch on.)
Here's 16 stock maps I would consider:
MM1FIR
MM2WOO
MM2FLA
MM3GEM
MM3HAR
MM3DW1
MM4DRI
MM4SKU
MM4BRI
MM4DC1
MM5STO
MM5NAP
MM6TOM
MM6YAM
MM6FLA
MM1DW1 (Thanks, Korby! I almost forgot this.)
MM7 maps purposely omitted.
As for custom maps, the majority of them are too large. (With the exception of "Duel" maps, which, ironically, are far too small for even duel. AMP01, XDUEL01 being great failures in that regard.) AMP02, IDM10 are fine for duel. Duel servers rarely host custom maps, so expect the overall familiarity with them to be low. A couple of other AMP / IX maps would likely fit although I haven't looked at them in long enough to say which ones specifically.