To be honest I also consider GemCraft to be one of the best TD games, followed by Immortal Defense (which has, that much is true, 0 replayability) while in Gemcraft you have so many choices and approaches that it is really fun to experiment even on maps you already finished.
My personal "favorite" exploration map is by the way C1
where I even got to a billion points, (this 1 run put me from mage level 321 to 2400 ) and to this day I can't replicate that on any other map. Though I regularly get to wave level 300, in reality if you want to really push for a good run you have enrage waves and that always
and to the extend that your defense can handle. At no point should your kill counter stop counting and this creates some really intense moments ;P
This idea of enraging waves is not found in any other TD as far as I know.
Also there is the infinite ongoing debate whether to use bloodbound or poolbound gems... bloodbound scale quite extremely up in the later levels, where your gems have a million hits or more.. but poolbound scale much faster and heavier earlier on only to patter out late game... and "more damage" isn't what you want late game, since your crit traps and tower are the one thing that kills enemies, if you don't have special bonus boost you end up with low damage no matter what.. so yeah; gemcraft.. awesome TD
But sadly really bad engine.