While it's not my favorite game in the series Mega man x 1 does probably deserve to be the most popular* game in the series.

Basically everything in x1 is just really good by mega man standards. Dashing and wall jumping make for more interesting platforming then classic mega-man, basically every character design in this game is great, the soundtrack is arguably the best in the franchise (let a lone the x series) and you have a pretty satisfying collection of maverick weapons and upgrades.

Only real complain I have with this game is that the Sigma stages feel kinda short and less climactic then the best wily stages from classic mega-man but overall it's just a really tight and fun mega-man platformer