I voted for Mega Man Legends off the strength that it's the series that most deservedly needs a sequel. The first game was "sandbox" before the pack-mentality video game media and marketing strategists started using the word. It had fully expressive characters, a gripping story, and successfully brought Mega Man to 3d, something not all franchises have been successful at. The sidequests made it a total package - like building and pimping out your own private clubhouse. Wonderful memories.

The original series might have the tightest gameplay (save for 7 and 8) and for that reason is arguably the best "games". But partially because of what I said above, I'm discouraged that Capcom is making MM10. The formula is set in stone, you could make a million sequels and they'd all be fun, much like MM9 was, but at the end of the day they really won't mean much to the medium of gaming on a whole.

I would recommend some of you play X5, it was intended to be the final game, and ties up some holes in the Mega Man Canon. It was also a tribute to the series, chock full of references for you true Mega Man lovers.