If you can get past the intentionally ugly thing staring at you in the upper-left corner of the screen the entire time while you work out puzzles, this game -- and its DLCs -- will captivate and consume you for hundreds of hours.

Tons of user-made content too, including a 2400+ room hold larger than all five games in this series combined, which just released after 10 years:
https://forum.caravelgames.com/viewtopic.php?TopicID=46328

I like challenging puzzles and played a whole lot of games in that category -- DROD is certainly one of my absolute favorites. Its gameplay mechanics are quite unique, it's mindboggingly challenging at times, it can be played in nice bite-sized "coffee break" chunks (every room is standalone and the game saves at every single move), and it even has some interesting worldbuilding, characters, and lore underneath all of that, which gains depth with each title (main DLC) in the series.

Normally when trying to describe a game, you would write something like "similar to ___" or "a ___-like", but, at least to me, DROD defies such categorization. Its combiniation of clever level design, precise gameplay mechanics, and variety of enemies and obstacles is very unique and allows for a huge variety of challenges.
Calling it just a 'puzzle game' or a 'dungeon crawler' or any of those tags it got on Steam doesn't do it any justice and I would in fact consider it misleading. DROD is a DROD-like, and so far, few other games can claim to even remotely belong into this category. You need to try it yourself (or watch some gameplay videos) to get a feel for what it actually is.
The graphics and voice overs can be considered an "acquired taste" by some I suppose, but personally, I enjoy them and I recommend to be undeterred by the presentation alone.

I played King Dugan's Dungeon many years back and loved it, but somehow lost sight of the series and forgot about it for a long time. Recently I rediscovered it here, and by now I played and finished Gunthro, King Dugan's Dungeon (DLC), Journey to Rooted Hold (DLC), clocking in at about 40 hours each (finishing most but not all challenges, YMMV), and I absolutely loved everything about the experience.
I very much enjoyed the addition of room challenges that are tied to Steam achievements. As someone with both, a limited frustration tolerance and no time to waste, I'd like to recommend newcomers to consider enabling the unlimited undo in the settings (unless you feel the urge to impress someone). The game makes use of discrete 8-directional movement, so get yourself a numpad or something equivalent.

While I have nothing to criticize about the games, I am not sure what this weird DLC setup is all about.
It certainly feels weird to have the majority of Beethro's adventures (who is the main character, after all) hidden underneath a Grandpa Gunthro title...
I would have preferred to have the main titles as standalone entries in my library (like The Second Sky).
In the end, the community did a great job explaining what order to play (and buy) the DLCs in (in the guides), and, after all, no DLC is needed to enjoy the title, as Gunthro and the Epic Blunder is a very good starting point in terms of difficulty and is pretty much standalone (prequel) story.