’90s-riffic
If you were an old school PC gamer in the ’90s, you remember Raptor. It was a really rad shoot-’em-up that had a heavy emphasis on upgrades, allowing you to slowly enhance your plane over the course of 27 levels.
It was a fully featured shmup for the time, especially since a number of eastern shooters only featured a handful of arcade levels with no room for an experience system. If you want to relive the magic you can do so with the 2015 Edition on Steam for $4.49.
Note that the “2010 Edition” is also on GOG.com for $5.99.