Some can simply be "different" each time you play. For example burbenog on expert has 3 different creep waves which can spawn each round. Adding something to "do" while your towers are killing stuff, i.e a hero/towers which can be used manually. Or different "races" so that you play most races on the same td.
The other successful ones simply need to be good for a very good reason. Power towers is utterly different to every td i've ever played but still keeps the idea of mazing. Mini TD literally being the shortest td i've ever played with its special "win" mechanics where you can only progress if another player is still alive and hard enough that quite often everyone dies.
But a great deal just grow old


My views on some others posted:
I really can't see how you can recommend poker td, it is ABYSMAL. I must admit i liked it for what 3 times? but after that, you simply can't finish it, it is SOOOOOOO boring, it's too luck orientated "oh look i gave him 20 gold and i got a good tower, i am good at this game" no you just got lucky.
I was never a fan of green td, was always pretty horrible for me, but other people have enjoyed it and it's lasted a long time.
i don't know how legion td can still call itself a td. it sort of it but mostly isn't. it's like a hero defense map but you have no hero and you can't control your units. WTF.

Soldier td is worth playing about twice only.
Wintermaul is ultra old and i always hated being with other players and someone will always screw it up.

Slams solo is good for a few games also, for it has the idea you can have as many creeps going through as you can kill. so some players on level 5 while others about 13. Is fun for that respect.
Line tower wars /agree seeing he's after a PVP element, definitely good, probably what he already plays.


If you wanted solo td's, try the ones i posted. Especially the ones i linked to, if you wanted team td's then ask someone else who played them. Only burbenog sticks out for me.

shopping maul imo..