I pretty much agree with BreathingMeat. My favorite games in the series are MM4 and MM5, which combine to form one giant game, World of Xeen. The only game that I haven't played yet is MM3.
You kind of have three sets of games in the series. MM1 and MM2 are really old and require you to draw your own maps and stuff. There are other issues that are a result of their age, such as only being able to save in an inn, and a high reliance on random monster encounters and random loot. These games can be fun if you are willing to put in the effort, but may be a bit too much hassle as a starting point.
MM3-5 are still fully turn-based but are more graphical and varied in content. Most people prefer World of Xeen out of these but I've heard good things about MM3 also. There are still a few old-school design elements present, like having to identify equipment (although you can just use the included cluebook to look up item stats instead... I recommend this).
Then there's MM6, which is fully 3D, and can be played in real-time or switched to turn-based mode. I found MM6 to be a bit of a monster slog, especially in outdoor areas where there are huge swarms of enemies. I liked the dungeon design a lot though, which is quite varied and uses the 3D engine to great effect in producing levels that extend in the third dimension. It's definitely the most modern and in many ways most accessible game in the pack.
I like the Xeen games because, while there is lots of combat, there's also a lot of puzzle solving, interesting places to visit and lots of little details and secrets to uncover. These different activities felt more balanced than MM6, which is mostly fighting (there's puzzles and exploration too, but fighting felt like the focus for me). I also find the graphics in the Xeen games actually aged pretty well, and are bright and varied. The game also does well in differentiating its various dungeons and locations, keeping everything interesting through varied looks and level designs.
So yeah, MM6 is the most modern so it might be a good place to start if you are new to oldschool RPGs. You might also try MM4 and MM5 for a different experience, which is more oldschool but still fairly accessible for the genre. Just try some of the games out! If you don't like one, you can try a different one and see if you like it better.
Post edited February 22, 2011 by Waltorious