MM6 is filled with dungeons everywhere. Downside is these dungeons are littered with trash mobs in every single room. Some of 'em consist of big open rooms with pretty much nothing in it. It's a very weird trip, though I liked it even after playing it a few years ago (so it's not nostalgia talk) because I just like myself some party-based hack n slash action, but it's objectively not amazing.
Puzzles are also much weaker in mm6 than in the former ones. I feel like the transition to 3D had some p.annoying consequences regarding the dungeon design.
MM1-5 is where you should go if you liked MMX, it's not really the same thing (MM1-5 feel bigger, and their dungeons are generally more varied, their systems are also closer to D&D) but then again, MM6 is absolutely nothing like it as well. Even MM1&2 are still more interesting to explore than most party-based hack n slashers I've played, and it doesn't limit you (well, besides the astral plane) in your exploration, given you can get passed the very monotonous "graphics". Isles of Terra or WoX is the best option.
Also, yeah, MMX got and gets booed, but it's a fairly good game. I don't like its systems though.
ps: if you want to play mm6, there's a mouselock unofficial patch. Strongly recommended.