6-8 are the good ones as far as I'm concerned. I started playing the series with 7 and I think that's the best starting point for new players. 7 has a straight forward beginning with a tutorial level and a tour guide NPC that will explain a lot of the game mechanics. In addition to the beginner friendly intro, 7 has; a large diverse game world, a huge number of spells and skills, a lot of race and class combinations, a clear focused plot line, superb music, a fun card based mini game, and a difficulty level that isn't too easy or too hard.
6 is awesome, but it doesn't allow the player to choose their character's race, features a less extensive skill and spell system than 7, and has a pretty vague plot line. It also has very little hand holding at the start of the game and is generally considered the most challenging of the 3d era Might and Magic games.
8 is a lot of fun, but it's definitely not as well rounded as 6 or 7. It brings in a new race system where some character types are vastly more powerful than others, dark elves and dragons in particular. It also has as a party recruitment system that, while cool, can make the game heavily unbalanced in the player's favor. It's possible to get 2 level 50 characters in your party early in the game and that kind of takes the fun out of lower level quests. Aside from those flaws 8 is a solid entry to the series and has interesting setting which is darker than the ones in 6 or 7.
A lot of people are really into the Xeen games, but I'd advise against them. I find them really hard to get into. Their game world is mazelike and can be pretty confusing and easy to get lost in. The gridbased movement can also make traveling to healers and stuff rather slow.