Let's start with Bloons. This surely must be one of the greatest TD games out there. There's a wide variaty of towers and updates, especially in the later editions. Most TD players know the game, but if you don't: don't look any further for a TD-game you want to play. Bloons 4&5 are awesome, but the earlyer versions were important for defining the genre to.
play Bloons 4
Plants vs. Zombies
Definitely a TD with an original concept, definitely one of the greatest TD's out there. There's smooth gameplay, great graphics, a good tactical idea behind it and humor. So what's the catch, you ask? Only a demo is available online. The real deal is available only through download and only free for a limited amount of time. Nevertheless, check it out, at least the demo! Oh yeah... plants vs. zombies 2 is coming soon!
Kingdom rush is another big name in the TD genre. There are 4 different towers available which you can place only on pre-determined spots. This limits your tactics a bit. However, the game looks great, has fun sounds, an upgrade menu and you have spells at your disposal. I can assure you, you'll have a lot of fun playing this game.
The game mechanics of Incursion are similar to Kingdom rush and just like Kingdom rush, this one is also a "must play". There are 3 types of towers which each produces 3 units. The units do the actual fighting, you can move them around close to there tower. There's a huge variaty of enemies and a fun storyline. Part 1 is a great game and part 2 a worthy follow up!
In cursed treasure you have 3 types of towers: demon, orc and undead. Each type can be placed only on a matching landtile. The game has upgrades, spells and a great design. Part 1 was better then part 2 in my opinion. However, part 2 is still a great TD game.
Onslaught has been around for a while now. It is your typical "place towers near a road and watch 'em pop enemies" game with one fun difference: placing towers near each other and fully upgrading will allow them to "work together". This way the towers will do cool combo's. You can also build your own levels. Try making a very long path and play that map if you have some time to waste.
Hostile Ally Defense
This game might be familiar to a lot less people. It didn't cause as much fuss as most games in this list. However, I feel it belongs in this list anyway. In this game you're able to control the incoming mobs, making them towers of your own. Sounds great, you say? I agree!
Penguins attack is as classic as TD's come. You can't be impressed with the originality that the games bring, but they are good in design and gameplay. Definitely worth checking out if you're looking for a fun standard TD game.
Does killroom belong in this list? Probably not... It is in here because it is a TD I created. Can you really blame me for some shameless self-promotion?
Enjoy playing, list will be updated...