I would like to put forward Armed Police Batrider, a completely bonkers vertical shooter by Raizing that’s got something like 18 or 19 selectable characters, including original ones as well as crossover ones from Battle Garegga and Mahou Daisakusen. You choose a team of three characters to play as (which given the game came out in 1998 I think must have been inspired by the King of Fighters) and then play through a series of unique stages that include bosses from all three games. It’s completely nuts and I love it. It’s also criminally unavailable in a modern M2 Shottriggers version, so you’ll have to grab a rom of it to try it out.
Since I’ve mentioned Mahou Daisakusen, I’d also like to mention that I’ve always loved the second game in the series, Shippu Mahou Daisakusen, which I believe has an outrageously expensive Saturn version but no other home versions! Dimahoo/Great Mahou Daisakusen, the third game, is good and extremely pretty, but I’ve always thought it doesn’t quite play as well or as interestingly as the second game. I’ve always wondered why no one else (that I know of, other than a completely sort of game like Roadblasters) attempted the shooter/racing game mashup to try and do it the right way, rather than the way that Great Mahou Daisakusen tries to (and honestly really fails at– the racing element feels like an afterthought).
I also feel like I should mention Crimzon Clover, which started life as a doujin game but now has both an arcade release and a Switch version:
…and Blue Revolver, which I haven’t played, but which has a super great vibe to it: