19. Black Station - Metro 2033
Metro blended a lot of different elements into one game and it surprisingly worked quite well. There was sci-fi, horror, action and stealth and on Black Station, you had to use your sneaking skills to a degree that would put Snake to shame. You had to traverse Black Station while avoiding a group of Nazis, and not the stereotypical World War 2 kind. Even the slightest noise made would set these guys on you. This level is also the last one to contain Nazis or indeed any human enemies. Also by eavesdropping on Nazi conversations you can gain Moral Points which will affect the ending you obtain at the end of the game.
18. Oasis - Serious Sam
Serious Sam was seen as Duke Nukem's mature, younger brother, but this didn't detract from the game and Sam himself being highly enjoyable. An evil, alien overlord called Mental has decided to travel back through time with the intent of wiping out Earth and it is up to Serious Sam to harness the power of the Time Lock to travel back to Ancient Egypt to save the day. In this particular level, Sam has to traverse the level while hunting for the last of four Relics which are needed to bring your spaceship to the present time period to stop Mental. After battling through hordes of beheaded Kamikaze zombies and bi-pedal robots you encounter a mass of killer Bullfrogs, the sheer numbers of them are enough to overwhelm you but with the help of your trusty Tommy Gun, you can squash them with ease.