I play mania because it's the gamemode I'm most comfortable and familiar with.
My first rhythm game was DDR Extreme 2, for the PS2. I remember getting it when I was young on some birthday (I don't quite remember which one it was) but the game came with a pad and everything. I was so excited because I liked getting new games (back then I only had like 2, and they were both sports games (Madden 2002 and some Nascar game)). Of course, not being a prodigy, I was pretty shit at it. But doing beginner songs, I slowly advanced through the difficulty levels until I got to the easier Standard difficulties. I remember playing classics such as Speed over Beethoven, Butterfly (Upswing Mix), and Captain Jack. I also remember advancing in the Dance Master mode, the "story mode" of the game, per se. Though I couldn't have gotten too far, I was pretty shit. Eventually I started to play the game less and less.
Now fast forward a few years (maybe like eight) and I start to develop an interest in rhythm games again. I pop out my old mat and begin to play, picking up where I left off. Playing more and more, I started to unlock more songs, more characters, and more courses. I was getting better and better now that I wasn't a little kid anymore. The first song I passed on heavy was
La Bamba, and I was so excited and proud of myself. Slowly, but surely, I passed every song in the game on either Heavy or Challenge. Well, almost all of them:
Cartoon Heroes (Speedy Mix) took me a bit longer to pass, and I didn't pass
PARANOiA Survivor 270 until recently. So of course, having my own money now from odd jobs here and there, I decided to collect all the DDR PS2 games; and I'm still on that mission; the only one I'm missing is DDR X2.
Meanwhile, I decided to branch out to other rhythm games. beatmania IIDX seemed like fun, but there were no machines near me, so I had to do research and get the next best thing, which turned out to be osu!mania. However, I ended up sticking to 4K instead of 7K or 7+1K because I found it hard to do, and I already had experience reading DDR. Getting used to having no colors and having to release freeze arrows was a pain, but I got over it.
tldr: Mania is the best osu mode because it is similar to DDR and DDR is the best rhythm game.