As one of the most classic arcade shoot 'em up video games from Japan, Raiden series definitely has the most and loyal fans players around the world. To a certain extent, AppNee can put it this way: Raiden is the rare peak work among all flight shooting arcade games.
Ultra Assault is an old, classic Raiden-style 3D flight shooting PC game released in 2003 with multiple high gaming difficulties. AppNee suggests it to Raiden series' fan players, you should not miss it.
DemonStar: Secret Missions 2 (short for DemonStar SM2) is the last expansion pack of Mountain King Studios' DemonStar series, with cooler graphics and higher difficulty than DemonStar: Classic, DemonStar: Secret Missions 1.
As the first sequel of DemonStar: Classic, DemonStar: Secret Missions 1 (short for DemonStar SM1) has not much difference from DemonStar: Classic in essence, but with several important and notable upgrades: an enhanced version of the original engine (arcade quality graphics, sound FX) and all new levels and music soundtrack.
DemonStar: Classic is a five-star vertical scrolling shoot 'em up style series PC game developed by "Mountain King Studios" (often mistakenly called "Raiden III" - a famous STG game released by Seibu Kaihatsu).
Raiden Legacy is indeed an excellent official remake version of the classic arcade game Raiden (雷電) series for PC and Android platforms. It includes Raiden, Raiden Fighters, Raiden Fighters 2 and Raiden Fighters Jet 4 editions in one game, which allows you to fully experience the nostalgia.