I gravitate more towards vertical shoot 'em ups since vertical ones like RayStorm, Touhou 06 ~ EoSD, and M2's port of Mushihimesama Futari ver 1.5 are got more into the genre, with RayStorm being my introduction during my childhood after playing a demo disc that featured it.
I'm not an expert on horizontal shooters but there are some great introductory ones I can recommend:
- Deathsmiles — Particularly its modern ports on Xbox 360 and onwards. The game can be very challenging but the modern ports feature various modes and difficulty levels that can ease things for beginners while offering challenging gameplaying and engaging scoring systems for experts.
- Natsuki Chronicles — I haven't played this one yet myself but I heard this one is a solid horizontal shoot 'em up that refines many of the issues that Ginga Force has (although it's also a very good slow burn vertical shooter even if all of its new ideas didn't quite pan out).
- Trouble Witches — The one doujin shooter that still hasn't gotten its sequel off the moon. It started originally on PC but it has seen various ports on Arcades, Xbox 360 (when it lasted), Steam, and more recently on PlayStation 4 and Switch. Its latest release, Trouble Witches FINAL!, I would say is probably the definitive version of the game yet since it has enhanced visuals, comes with all of the DLCs from the Steam version, features with new modes, another playable character, and online co-op after a patch update (sadly no local co-op like in NEO!).
- Fox Flare Night — An upcoming shoot 'em up by Outside Sotonohito, and I think still has a demo available on Steam. From what I played, it seems to build off his previous shoot 'em up games and features an optional shop system where you can power up Mio and her abilities. This one is a bit on the lewd side with girls with enormous bust sizes, but if that doesn't put you off, I'd high recommend checking the demo out.
- Platypus Reclayed — A remake of cult classic PC shoot 'em up I discovered a while with amazing claymation visuals. I only played the demo of this and it's kinda tough, but it's very fun and it has been getting some great updates recently too, adding quality-of-life improvements, rebalancing the difficulty and making this more approachable for beginners, a free Halloween-themed DLC, and official modding support for the Steam version.
- Earthion — Ancient's latest game and newest Genesis game* since Beyond Oasis, it takes inspirations from the likes of R-Type and Gradius but I feel this one is more approachable since it doesn't punish you as badly for taking hits (although losing lives can damper your chances of coming back), offers various ways to strategize your ship's specs, different weapons to experiment with, and the difficulty on Normal at least isn't too demanding. The Genesis version isn't available yet but you can get it on PC and current consoles.
