Armored Core 4 (Playstation 3, 2007)

If you liked the previous Armored Core games (I have 2 and 3), you'll undoubtedly enjoy this one. The same familiar controls and gameplay style with revamped graphics will make it easy to jump right in and enjoy the heck out of it.
For players new to the AC game series, the handling of your armored core is not hard to get the hang of. After the tutorial and one or two missions, you'll be blazing around like nobody's business.
Customizing your unit's design, color scheme, and individual parts (head, body, weapons, leg-type, etc.) will offer plenty of gameplay alone. For those who don't really care much about customizing their vehicle (but really, who doesn't?), it's simple to play right through with one of the default designs, though you may want to at least upgrade armor and weapons occasionally. =)
Overall, this game is (in my opinion) going to be most rewarding for previous players of the series, but as I noted, it'll still be awesome for anyone who's never even heard of Armored Core. Good luck!