I also recommend the Kingdom Rush series! :)
Kingdom Rush
Kingdom Rush: Frontiers

And as tremere110 said, there's Kingdom Rush Origins finally being released for PC in a month or two, hopefully we'll get it here on GOG.

I would also recommend a game that is not quite a Tower Defense game, but something that may be enjoyable to those who like Tower Defense games: Infested Planet

Edit: fixed link.
Edit2: I missed Amok had recommended Infested Planet already, so my post is a second vote for that game. :)

Bloons TD 6 is out on android and it's rather fun, a bit different with heroes as well as terrain obstacles that can impact the range of units like the sniper monkeys. Well worth the money.

For me it would be:

GemCraft: Chasing Shadows
Orcs Must Die
Defender's Quest

probably in that order. Kingdom Rush and Anomaly enjoyable but not on the same level.

There's PixelJunk Monsters here for sale. It's a fun charming little game.

I don't really have much to add to this thread, as most/all the standout titles have already been mentioned. But there's Super Sanctum TD, a traditional isometric TD game, made as a de-make to the Sanctum 1&2 games, which were recommended by amok.

I liked it a lot, though it doesn't bring any innovation to the genre.

Also, shame on you amok!!! You recommend Creeper World series, but you don't mention Particle fleet(dev site, steam)? SHAME! ;)

I consider Particle Fleet an RTS, not a tower defense game. it is good, though.

Honesty, I view Particle Fleet as a natural progression to the Creeper World series, so it never occurred to me that it might be considered to having jumped genre. I see how it might be viewed as a RTS, but after Creeper World 1&2&3 it will always be a TD for me.

As others have mentioned, Gemcraft is fun and is available on armorgames or kongretate et. al.
Links:
http://armorgames.com/play/1716/gemcraft
http://www.kongregate.com/games/gameinabottle/gemcraft-chasing-shadows

Plants vs Zombies 1 is old, but fun. The second version is on phone only and I have not played it yet.

Yes, I love that game, I wish it was available here on GOG.

Defense Zone series.
Ancient Planet.