(The game I'm eyeing up, both figuratively and literally, by the way, is Osawari Detective Ozawa Rina Nameko Rhythm, a wacky 3DS spinoff of the Touch Detective series.)
I'm going to do my best to devote some time to the above-mentioned title this weekend (and then share my impressions here next week, of course), but in the meantime I thought I'd write up a few blurbs about some of my favorite rhythm games--games I'd highly recommend to anyone who is keen on participating in Anne's game-along but hasn't a clue as to what to play.
HarmoKnight (3DS eShop)--This choice could be called controversial, considering I wasn't entirely enamored with the game after my first encounter with it, but I still think it's worth checking out if you're looking for a vibrant-looking game that combines elements of the platformer and rhythm genres. I probably wouldn't make it my first #RhythmMonth pick, though, unless I'd already played the other titles discussed in this post.
Space Channel 5 (Dreamcast, PS2)--I've had a ridiculously good time playing the original (Dreamcast) version of this game over the years. The soundtrack is stuffed full of songs you'll be humming long after you power down your system, and the aesthetics are as pleasing as any Sega has produced in its long and tumultuous history. Should you not have a Dreamcast or PS2 handy, or should you simply not want to invest in those iterations of this title, though, you can't go wrong by picking up the sequel (shown in the screenshot above), which has been available via PSN, Steam and XBLA for ages now.
Theatrhythm Final Fantasy (3DS, iOS)--Based on everything I've heard and read so far, you'd be wise to purchase and play this game's Curtain Call follow-up rather than the original. I've yet to play the just-released sequel, though, so I can't say anything about it here. If it's anything like the first Theatrhythm, though, it's a thrilling addition to the genre that deserves the attention even of folks who aren't Final Fantasy fans.
A handful of rhythm games I've heard are good but have yet to play myself: Gitaroo Man (PS2/PSP), any/all of the Hatsune Miku titles, Maestro! Jump in Music (DS), Magical Beat (Vita), most of the Taiko no Tatsujin titles and Vib Ribbon (PS1/PS4/Vita).