For solo:

BBlair - I guess it's just clear that this level is handy for all amount of players, and the duels should be the most exciting ones.
Bleached Diamondus - This is a level that tests the cunning side of players in duels, it's sort of small and fits quite only for duels(reminds me about the map 'Plunder Isle' in WC3 ROC that is the only map in the solo-pool made only for 2 players)
Happy Semiconductor - Offers multiple ways for domination over duels, lets players improvise a lot, really
Knightmare Castle - This one should be definitely for duels since the bases become hard to exit with more players. Also there are simple solo-tactics to dominate this one over your opponent
Sanctuary - Not completely sure about this one but it could be added into the pool for a tentative trial
SILWI - This one is a good duel-level especially for those who prefer being Jazz and don't mind warp-pits

Suggesting rest later...