Back when I took Halo pretty seriously (Halo 1/Halo 2) there were very little resources for players to watch and learn from the best in the world -- nowadays? The top pros are all streaming on a daily basis. If you want to get good at Halo 4, and learn from the top players in the world it's relatively easy.
Back in the Halo 2 days, my favourite player to watch was Dave 'Walshy' Walsh -- a player known for being lethal in close encounters. Walshy is mostly retired now, but he recently posted a great clip of him playing with a host of younger pros and, whaddaya know, he's still got it!
I still have my reservations about some of the new multiplayer mechanics being added to the Halo series, but it's great to watch some of the best players in the world explore new systems and figure out how to adapt.