The original for me, I actually finished it a couple of times. You just have to memorize the maps and learn where to use the right keys. Great game really.
AB3D have got some great levels, it had transparent water that you could walk around in before Doom had it. Its been some time since I last played it but it was a solid game if you compared it to Doom, but yes the graphic sucks.
Tower assault is IMO to hard. Have anyone finished it without cheating? I hate when good games are too hard.