Mine was cinderella phenomenon. I have no recollection of how I stumbled across it. I was obsessed with Ouran high school host club as a teen so it probably looked intriguing to me as second time I'd seen a "reverse harem" on a cover. I played the entire thing within a single weekend. Then... I didn't play another otome game until Piofiore came out on the switch. No idea why I didn't. I vaguely recall not being able to find any other free games that were of similar quality, but I was an adult when Cinderella Phenomenon came out so it's not like I couldn't buy things online.
Anyway, like many others I bought a ton of switch games during the pandemic and one of those was Piofiore; I've been playing otome ever since.
My first otome (without really knowing what a otome game was) was Princess Debut on the DS. I was intrigued but got too busy so I’m not sure if I finished it. I do remember the “dancing” part.
My first otome that I did finish was… Hatoful Boyfriend, because I’d heard of its hilarity (and Shuu specifically). The BBL (not brazilian butt lift) route made me interested in visual novels as a whole (only other exposure was Phoenix Wright) and I then picked up Hakuouki for 3DS… the rest is history. (And my enduring love for Okita and hoping he’ll be okay.)
I was very surprised and impressed at the quality of Cinderella Phenomenon when I played it.
There were more but sadly not localized in English. There are some fan patches for Tokimeki Girls’ Side, the Prince of Tennis otome games (2 of them), and Love Revo and recently the Vampire Knight otome (for DS). All other English translated otome sadly went to PS Vita/PC at the time (my reason for getting a PS Vita other than Danganronpa lol). I would have loved more English localizations for DS/3DS…