IMO most of the Mega Man games had great music, with the exception of maybe MM7 (and some songs in Rockman & Forte/Megaman & Bass). Even MM8, considered a black sheep by many, had some really great tracks. The MMX series had awesome soundtracks too, but dammit, MMX2's soundtrack can tie itself up with bricks and jump off the Golden Gate Bridge for all I liked it. One of the worst soundtracks ever made.

Anyway I'm probably one of the biggest Mega Man (and Capcom) fans-- ever and this series in particular helped me get through a lot of tough times, so it's near and dear to me, much like Street Fighter (and if you doubt my fandom, you will soon see ;D).

My favorite Mega Man series, however, is not the original, but Legends. Legends was one of the best 2d-to-3d adaptations released at the time, and although it wasn't completely a Mega Man game in every sense of the word, it had many traits from the original games. Basically it was a little dude blasting the hell out of robots just like the old days, and done with great characterization, gameplay, and weapon selection. Just didn't get better than that.

Like Bolivar up above me said, it introduced a 3D sandbox world before most other games could even think to. The story part of the game was convoluted at quite a few points but mostly understandable once you put all the pieces together. Just remember: all the humans in the MML world are artificial humans called Carbons. If you remember that the story makes a lot more sense.

The characters were very well fleshed out, especially the Bonne family. Tron was a very believable rival for Rock, and had the ironic tinge of actually being in love with the guy for saving her from an angry dog named Paprika in the first game. Teisel had some crazy one-liners and was just overall comic relief. Bon was essentially just a big lovable baby.

There are many other notable characters from the series, too, such as Klaymoor, Glyde (Glyde later got his own NetNavi form in Battle Network), and Sera, who are all pretty much rivals to Rock throughout the series. Don't forget the main characters, like Rock, who have real convictions and motivations for digging, and Roll, who is a generally sweet and helpful character.

Voice acting was great, especially for a game from 1997/1998. Much better than Mega Man 8 and Mega Man X4 by far. Dang near every line is well acted and makes you emphasize with the characters that much more. I do remember all the crying about Volnutt's voice actor from the English version of the first game (Corey Sevier) being changed to Susan Roman for the second game.

Gameplay was just addictive. I mean, in the first game, having to use the L1/R1 buttons to turn was awkward at first but then you got used to it. What made the gameplay in the MML series so fun are three things: the weapons/accessories, the ruins, and the enemies, especially the boss fights. I'll touch on this at a later date.

The Legends series was mostly overlooked after the first installment. The first game sold well in Japan and sold well enough in the US to get the Greatest Hits label, while the second did moderately well in Japan and not so well in the US. Misadventures of Tron Bonne was a lead weight in the US due to Capcom lowballing the amount of copies but from what I heard did well in Japan.

In the past few years, other than some cameo appearances from the Legends characters in games like the PSX version of Pocket Fighter (Rock Volnutt/Trigger and Roll show up in the background of the Running Battle mode), Namco X Capcom, and Tatsunoko vs. Capcom, where Volnutt is playable, not much mention has been made of the Legends series in past years. Its relatively small fanbase compared to other Capcom series and lack of more MML games that weren't for mobile phone are probably to blame.

Capcom did release 1 and 2 for PSP and were gonna put it out in the US but they were rejected by Sony, so no go there. There have been a few cellphone games, including one MML game that could somewhat be called a sequel, or at least a prequel, but they are all Japan-only.

However, the creator of Mega Man, Keiji Inafune, has stated a desire to do an actual third Mega Man Legends/Rockman DASH game, but has stated that the budget he'll need to do it is quite high (IIRC, it was something like US$15,000,000). So while there's a glimmer of hope for the franchise to come back, the hurdle that will need to be climbed is a high one indeed.