I can't really recommend one since I haven't played Grimrock and M&M not so much. However, you can't do anything wrong with JA(2) imho. What Sir-Tech games made so special for me is their love for detail, sense of humour and soul and I haven't found games close to that from other companies yet. + I don't know much people (erm, anyone?) who like their one serie but dislikes the other.

Regarding M&M I have only played M&M6 and despite the masterpiece-status I never got into it. It's hard to describe, but this game just felt totally generic and uninspired to me. It probably got all the ingredients but presented in a way that totally turns me off.

Jagged Alliance is a bit different beast altogether, and Legend of Grimrock is real-time only.

Might and Magic I-IX - choose it and you won't regret.

I had no idea M&M had sci-fi elements. Seems pretty interesting. I'll pick them up with one of these sales.

I'd vote for... Wizardry 8.

That said JA2 is a masterpiece without doubt. If you like tactical shooters it's only a question of when to get it imo.

The Legend of Grimrock games are based on older tile-based dungeon-crawlers like Dungeon Master and Eye of the Beholder, and as such they are real-time, although not fast real-time, and they emphasize in-world puzzle solving at least as much as combat. The story elements are presented by implication more than direct narrative, similar to Wizardry 8, and there's a similar ability to play with wildly varying parties, although you won't find any NPCs to interact with (except by way of messages left behind). You're reasonably likely to enjoy them if you like the Wizardry series, but they aren't a direct analogue.

I actually did not like legend of grimrock that much. It had too much down time and too much tactical kiting type of combat. Wizardry 8 had nothing like that. Might and Magic is fairly close, also the D&D would be similar with skills and turn based combat.

Jagged Alliance 2 (with the JA2 1.13 mod installed) is a tremendously fun game. Turn based tactical combat with real time exploration. Arguably the best RPG that Sirtec published.

Legend of Grimrock has gotten a lot of positive reviews but, though I bought it years ago on Gog, I cannot play it because it is real time button- mashy clickfest combat which I do not enjoy. If you like games like Dungeonmaster and Eye of the Beholder then I imagine you will be at home with this game.

Might and Magic (I recommend 7, then 8 as M&M 6 is not as in depth or polished) games are also very good but they are also very different from the Wizardrys mechanics-wise. If I had just finished Wizardry 8 then, knowing what I know I would probably play Jagged Alliance 2 next (as it is also a nice change of pace from science fantasy) then Might and Magic 7, then M&M 8.