I like D2 Nysa, but you might want to double the primaries so there isn't just one Gauss. I'd prefer it with low ammo regardless, but YMMV.
Others...
D-Day '97: I've heard positive things said about it, but haven't tried it in 1v1 myself.
Judith: was OK.
Nocturnal: was good.
Pyromania: appropriate if you want a heavy gauss+mercs level.
Reason: definitely good (in D1 as well, although it gets played less these days).
Vamped: as good in D2 as anywhere else, although it does play differently there.
UES: if you're insane enough to try it, 1v1 shakers might actually be fun in this map.
Some of DH's recent levels are also listed as appropriately sized for 1v1; I don't know if they've been tried yet, but if you want to do blind matches they might be good places to start.
Argonia (low weapons, basically a plasma dogfight level):
http://enspiar.com/dmdb/viewMission.php?id=1060Inhaling Helium (Gauss 'n' Mercs):
http://enspiar.com/dmdb/viewMission.php?id=1062Year of the Horse (pretty standard all-around):
http://enspiar.com/dmdb/viewMission.php?id=1065