Halo Combat Evolved

The first entry in the Halo series and it still plays incredibly well from a singleplayer perspective. The multiplayer I would avoid though since it is quite inaccessible especially if you have never played it before.

This game has many of the weapons and mechanics that persist throughout the series. It is certainly the roughest of the original games but is absolutely worth playing. The levels are varied with everything from night time stealth missions to tank battles. The combat is fun and there are plenty of approaches to most situations. I think this one is most fun on heroic difficulty, normal felt a bit too easy and legendary could be frustrating at times. Heroic is a nice middle ground, and adding a few skulls, which are difficulty modifiers, can be fun to spice it up a little.

The Master Chief Collection is the best way to play this now.