I prefer stories situated around action, characters and plot, meaning that my betareading style is usually to try and make stories more intense and dramatic. If somebody needs a betareader for a story I at least expect them to have some experience with writing and the ability to weed out harmful problems they can readily see. I also don't like people trying to write stories where most or all of the words are mispelled, grammar is thrown to the winds, etc. That is not a good fanfic, that is being lazy. I will, however, make some exceptions.

In terms of strengths, I work best with stories involving action & adventure, and sometimes comedy. While I have not yet perfected any of my work, or even come close, I strive to see well-developed characters in a story, even if I cannot always reach this goal. When betareading I look mostly for word-choice errors, plot holes, spelling problems and general grammatical errors, though I can't always find them all.

I am terrible with drama, can't write worth a crap in terms of romance, which I'm afraid I will never understand, and angst is troublesome. As such, it would be nearly impossible for me to help you with such stories, but if nobody else will help I will at least help you with spelling and grammatical errors.

Anything involving heavy fighting, danger, suspense, well-developed characters and a good plot. I especially prefer last stands, unwinnable situations and the occasional suicide mission. Generally, the more violent and twisty the plot is, the better. EDIT: fighting is not entirely necessary. Just a good conflict, whether symbolic or real, will do.

Romance, drama, and angst, are the 1st/2nd/3rd easiest-for-me-to-dislike stories, in that order respectfully. I won't deny that you have many more options and often drama and angst make for very good stories, but that doesn't mean I can understand them or work with them in any way other than to say "good plot, nice word-choice" when the situation calls for it.

Game Assassin's Creed

Game Gears of War

Game Pikmin

General Adventure

Parody Sci-Fi

Fantasy