MDK is an action-adventure platformer game in the third person: it's pretty frantic shooting action, primarily against alien foes and their death machines. The alien architecture is uniquely gorgeous and actually alien.
What makes it awesome is that it's just generally fun as hell: you have a machine gun and can fluidly strafe, you can spontaneously snipe with full miles away zoom, your jumps are augmented by an easy to deploy and really sleek parachute, and so on. It's also by the Earthworm Jim guys(Shiny?) so it's generally funny as hell. The first one is a bit primitive but solid.
... But the second MDK is a masterpiece. The humor and writing are ratcheted up and polished, there are two extra playable characters, the pacing and challenge is awesome, and I bet you lucky pussies can adjust the difficulty for the PC versions. It's gratifyingly hard and one of the most beautiful and tricky platformers I've ever played, the other two characters are fun as hell(Max is EVEN SHOOTIER, while they made Kurt a bit stealthier and sniper dependent to provide a nice mood back and forth. The Doctor's first level is kinda hard but his play-style is totally unique and he has the best and funniest writing)... and the boss fights.
Oh God.
The Boss Fights. They're so good. They're so robust. They're so fun.
MDK2 is one of my favorite games of all time and the finale is absolutely brilliant(you can choose which of the three to beat the game with and all 3 have a unique boss fight experience + stage + cinemas for it) and I can't recommend it enough.