It's pretty difficult to rank the MMZ games, they all have their strength and flow, they're all the same, but at the same time, they're all different.
Anyway, MMZ has the best overall design, the mission system and the whole map is really great, there's leveling and all, which I don't really like, but there must be some fans over here. It's also the hardest one.
MMZ2 has some really varied levels, no leveling shit, and awesome armor system, and it feature the best weapon (chain road > *). The boss are slightly better than in 1.
MMZ3 finally has a great elf system, the levels are really good, and the overall game is great. There are some really good armor parts such as the shadow dash, the auto charge, and other stuff that makes the game really fun. And finaly we can skip cutscene and all. All in all, I'd say, Z3 has the best gameplay mechanics.
MMZ4 has the best levels I've ever seen in a Megaman, and maybe in a plate form game. It's just insane, the big canon, the magnetic stuff, the level with the sun burning you, every ****ing level features and awesome idea. I was blown away by the level design of that game, the weapon system is pretty good if you take the time to experience the different weapon you can steal. The is a little easy, but there's some awesome boss battle (like in all MMZ) especially that Craft bastard which is one of my favorite boss. The scenario seems a little off topic, it doesn't really respond to the mystery leaved by the previous games, but who cares about scenario in Megaman anyway? It also features the awesome parts of Z3.
All in all, I'd say that 2>=4>=3>=1 but they're really about the same quality. They're all insanely good. My GBmicro will never die thanks to Inticreates.