STRESSFULNESS AND QUALITY OF GAMES:
Disclaimer: the next part is about MM6-9. I have never played 1-5 and thus, I pretend they dont exist:
In terms of stressfulness, I would say that MM9 is the most stressful MM-game (so much inventory management, money issues and long winded, buggy main quests), followed by MM7 (game turns into a chore after you make "the choice").
MM6 and MM8 are the least linear and best MM games in my opinion (I know people love MM7 and like to throw "mehs" everywhere at MM8, but I think it's 8 is better than 7. I think the story of MM7 is really bad (especially the incredible cheesy plot twist that forces you to work with the advisors near the end). The story of MM7 is about the same quality as that of MM9.
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MM6 has the best story, has the best soundtrack, has the best world (actually fun to explore) and is thus, IMO, the least stressful game.
I rate MM6-9 from best to worst:
I am not familiar with the later games, but I *am* familiar with 2-5, and I have seen videos of the NES version of 1 (but wouldn't consider myself familiar with it, as I haven't actually played it).
In terms of linearity, it seems that the linearity decreases as the series progresses through these games. In particular, 5 is less linear than 4, as many areas are not so easy to explore at low levels, most areas in the game require specific items to enter, and the endgame (the portion of the game where the non-linearity stops and you have to do things in a specific order) is longer than in other games in the series.
What I can say about the games is this:
MM2: Has the best combat of the early series IMO (the only one that could possibly compare is MM1, actually). In particular, you can actually see things like how much damage your attacks are doing, and spells feel worthwhile throughout (unlike in 3-5 where they're not so great at higher levels). There is a transportation network that lets you easily reach all the towns early, and you can get access to spells like Teleport that let you reach various key locations easily. (As a side note, the Etherealness spell is more useful in this game than in 3-5 because the walls are thin rather than a full square wide.) One other nice thing is that encounters respawn and events are repeatable, unlike in later games in the series. There's also the fact that *all* aging is reversible, not just magical aging. The main problem is having to split your party for mandatory quests. There are also some enemies that can destroy *all* of your gold, gems, or food, and that I believe can even steal your items. Also, this game has the nastiest troll I've seen short of a "dead person walking" situation or deliberate softlock; if you take Murray's treasure, you are punished harshly for it, and the game does not make it apparent right away.
MM3: Has some interesting spells that didn't make it into the Xeen spells (albeit Half for Me's implementation has a couple issues, one making it less useful for its intended use, the other allowing for unbounded exponential HP growth (watch out for integer overflow!) of the character knowing the spell). The biggest problem is that one of the necessary spells, Walk on Water (expecially important in a game where the world consists of islands), is not learnable by anyone in the default party. I actually consider this game to be less stressful than the Xeen games, even if a few areas can be nasty (hint: an enemy attack that fails to do any damage can't do anything else, either; try Power Shield if multi-target death attacks are posing an issue). It's also interesting in that you need only slightly more than a third of a specific key item to complete the game, meaning that you can choose which areas to explore and which to skip.
MM4: This game is different in that your level won't rise that high; you can't train past level 20 (without going to the Darkside, at which point you're no longer playing MM4). The slower scaling means there's a bit more freedom to exxplore, but unfortunately many dungeons require items from previous dungeons to explore, making the game structure consist of a couple main linear "threads" instead of being fully non-linear. One interesting aspect of this game is that Teleport works nearly everywhere, and is in fact mandatory (which makes the fact that druids and rangers don't learn it even more vexing, as it further limits what parties can beat the game without getting lucky with random item drops).
MM5: Probably the most stressful of the early games, actually. As I mentioned before, most areas, including even the towns, require certain items to enter. If you try to explore, you will encounter some very strong enemies (like those armadillos; if you've played this game, you know what I mean). Furthermore, while you have control of the order in which you do the towers and a few other areas, you still need to do most of them, and from the time you restore the castle (arguably earlier, starting with that labyrinth with all the minotaurs) to the end, the game is linear. (One oddity is that Castle Alamar, where the end of the game takes place does *not* require anything to enter.) There's also issues with gold supply later on; the game showers you with millions of experience like it's nothing, and it's even possible to train up to level 200, but the game basically stops giving you gold needed to train, while the amount gold needed per level continues to increase (while XP requirements do not). If you go for the World of Xeen ending, the problem becomes even worse, as there's a dungeon that requires millions of gold just to not take absurd abounts of damage, and if you've spent your money in order to actually make use of the XP you've earned, you won't be able to pay this.
In any case, I would say that MM2 is the least stressful of thest 4 games (even with certain nasty enemies present), while MM5 may be the most stressful.