It depends on if your intent is writing for enjoyment or money.

If it's money then you'll want to review popular games that are short on reviews.

Google has a various tools to let you check the popularity of searches (have a look at the "Search-based keyword tool" and check that against the amount of reviews already out on the web. Optimsing views would mean checking the that it's a massively popular search, with very low competition. Not always as easy as it sounds :(

If your writing mainly for enjoyment then go with the games you love, and already play.

I like shoot em ups and realistic military genre games myself. Something like HALO for shoot em ups and Medal of Honor or some of the similar military types.

Review games that are popular or just coming out since this will bring you more views. Makes no sense to review old games. Check out some vidoe game review sites for some ideas. Such as Gamespot, IGN, or Gamespy.

E3 just happened, I would recomend highlighting the new stuff that has been released with links to Youtube trailers, first looks, and gameplays. Dead Isle, Halo 4, Modern Warfare 3, Battlefield 3, Uncharted 3...etc the list goes on. But really, PC games are pretty big too, and all you have to do is look on Steam to find games that a lot of people are playing. Terraria and Minecraft are big now, but are probably saturated. Subscribe to commentators like Seananners and xJawx on Youtube to find out what they're playing, and review those as they have huge followings (muzzafuzza gaming would be another one looking at). Besides that, just keep at it! The more you work, the better you'll do.

I'd recommend games that you want to review.  It can be new games or old games depending upon what type of audience you're looking to get.