For solo TD's (so you don't need to rely on allies):
islands td: absolutely great solo mazing TD. hasn't been played in ages
Power Towers: A different TD, you either like it or you go "wtf is this crap" and never play it again, i'm the first type seeing i'm recommending it
Minotaur TD 2: Completely unheard of for most people, but seriously, try it. Innovative levels, an "extra" creep spawner, different difficulty levels which'll keep you trying.
Element TD: Not a mazing td, but THE best one i've tried which didn't include mazing.
Gem TD: Very challenging and weird td. You've probably played it tbh. Still very cool.
Mini TD: Really simple but actually really fun, really fast so if you've only got ~10 mins you can usually fit a game of this in. Very old.
Sprout TD: Again a different idea, but a good one. Definitely worth a few tries.
if you would like any of them here are the links:
islands td: not entirely sure which one works now, as several are bugged as they are old.
Power Towers:
http://www.epicwar.com/maps/123186/ (PVP IS ON THIS MAP)
Mino TD:
http://www.epicwar.com/maps/5961/ (PVP IS ON THIS MAP)
Element TD: Should be hosted on battle.net
Gem TD: look bnet.
Mini TD: Don't know which version works for this one now
Sprout TD:
http://www.epicwar.com/maps/81512/ (ALSO HAS PVP)
Element/gem td will come up pretty fast on epicwar/google searches.