For those who like tower defense games, which ones are your favorites. I am curious both for what is available on GOG and then on other platforms. I get into these when I need a time waster.

The Kingdom Rush series
The Bloons TD series
Several flash others but flash is now sort of dead.

EDIT : The Orcs Must Die series although it's not a TD striclty speaking, but a hybrid FPS / trap-laying defense game.

Wow! Thanks. They all look great. I don't have any of them yet, but I have them wishlisted now. Thanks for the recommendation.

Bloons TD 1-5 can be accessed freely through The Ninja Kiwi Archive which is an embedded flash player providing acess to all their old flash games (SAS Zombie TD among others).

I think you mean TPS (it's third person view, not first, isn't it)? Sang-Froid also falls into that hybrid category, and I really liked it. Clever game, nice atmosphere. I actually really hate Tower Defence games, so a hybrid like this is the only way to make it fun for me. Well that or a reverse Tower Defence (Tower Assault?) like Anomaly. But even that got old fast for me.

Defender's Quest. 100% the best one.
Infested Planet
Anomaly series (a pseudo tower defense hybrid; just pick one).

Worth checking out:
X-Morph. It's an odd tower defense/bullet hell game. With couch co-op each person taking one of the roles and freely swapping.

One NOT worth it at all:
Gemcraft. I have no idea how it was so successful to spawn sequels. It's brutally, perfectionistically, difficult, requires "grind" to level for progression... and one of its core mechanics makes no sense (impure 'mixed' gems work better than those you work hard to maintain a single color).

So I'm going to admit: I really don't like the ones that are all about "build the maze" and antics like sell wall, build wall elsewhere to loop them and all that nonsense. I prefer those that are largely "prepare and watch on an existing track".

(One final edit)

Monster Train is absolutely "Tower Defense" adjacent. You set up a gauntlet and hope to kill all the foes before they kill your heart. But it does it through the trappings of a turn-based deck-building game. Definitely recommend it. (Especially if you've liked Slay the Spire or ilk before.)

Defense Grid: the Awakening is a cool TD game, it was free on Epic at one point (Christmas 2020?) and support touch inputs. It's tied to Epic's client though, probably becasuse score leaderboards.
The main campaign is interesting, with some maps being very open and prone to try new stratagies, while others act a bit more like a puzzle. Each map has a few fun challenges to keep you busy.

There's also Defense Grid 2 but have no interest in it due non-touch support

The Bloons series, definitely

Kingdom Rush definitely, from what I've tried. On PC though, not the mobile version.

It has to be X-Morph. It combines tower defense with action, as you also control a small ship to attack the enemy, it adds another dimension to the tower defense genre, and the combination works really well. It's a cool game in itself, with a fun and entertaining storyline-concept.

The Anomaly series is still fun.

Empires in Ruins is a fun and unique mix of 4X and tower defense, with a cool atmosphere/setting. It's not on GOG at the moment, but there's a demo available on Steam.

Cursed Treasure: Don't touch my Gems! and its sequel are really good.

Look up Infested Planet or one of the Anomaly games. They're games where you're on the offensive, but are still tower defenders. They feel great when you tech up to be squashing hordes.

I actually really hate Tower Defence games, so a hybrid like this is the only way to make it fun for me. I suggest to visit bristol 8rental.com/rent-car-driver-bristol

The best tower defense games ever made (completly objectivly!)

Defender's Quest
Immortal Defense
Defense Grid
GemCraft - Chasing Shadows
Bloons TD 5 and 6
Orcs must die
Revenge of the titans
Cursed Treasure 2: Hey! Heroes! Leave us gems alone! (there is a non-web version on Steam)

Not quite TD games, but honorable mentions:

Creeper World series (https://knucklecracker.com/)
Infested Planet