The Commando Series has some of the greatest diversity in bosses, enemies, environments and playstyles I’ve ever seen in a flash game.
It is THE quintessential side-scroller Shoot 'em Up flash game. It’s basically Contra, if Contra had a gajillion weapons to choose from (at least from 2 on), extremely smooth gameplay, and fantastic graphics. I know that Commando 1 (somehow mislabeled “Commando 2” on this site) is here, but the sequels is where the series really shines.