I have to say Halo is the better of the 3. I've played all 3, and must say they each have their own strong points. I like Halo, because it's so much more than a game, there is a whole comic series on the Lore of Halo extending past Halo 4, that's how the future games come to be. Aside from the Lore and comics, Halo has a pretty awesome campaign storyline (If you payed enough attention, all the little details go together) Something that both games lack, GoW less than CoD is love AFTER the game is released. Halo 4, every week since it came out released 5 Spartan Ops missions, added a new playlist in Multiplayer, and do Daily Challenges to earn more Credits. I know Gears of War does something similar, but CoD from what I have seen, does nothing in the similar. That's why my vote goes to Halo.
Guitar Hero is always a good game for the Xbox. If you're into RPG's then Lost Odyssey is pretty awesome or even Final Fantasy 13 part 1 and part 2. Like others have also suggested Minecraft and Castle Crashers are excellent choices as well, there are tons of great games you can buy off the marketplace that are T rated and below.
Darkrai is faster, so the whole Dark Void and putting to sleep thing would work, also, Darkrai can learn Ice beam I believe, so that would kill Girantina, easy.
TM and HMS Are neverending in pkmn b/w, and yes there are move tutors to remove the moves, just gotta look for them, they're all usually in the main cities, I beleive if you look at the map it tells you under the description
