10 > 3 > 2 > 5 > 9 > X > X2 > IV > R&F > V > 6 > 4 > III > 7 > I > X3 > X8 > 1 > 8 > X4 > X5 > II > X6 > X7
I haven't actually played X7, but there's no doubt that it would go at the bottom.
Everything else is basically ranked by how much I like it. I included basically every noteworthy Mega Man platformer in the classic and X series, I don't think I missed any. (Wily Wars isn't important enough to rank.)