I read that as best selling and fastest selling at this point, not as bestest Halo ever, look at us!
Regardless, its the most consistent fun I've had in multiplayer. Netcode is solid, JIP works as advertised (works like a dream for me, but your mileage may vary), and the core movement and gunplay is the best there is. Campaign is gorgeous but less cohesive than others in the series and a letdown story-wise, and Spartan Ops is good as a terrorist hunt/co-op elimination mode, but needs more varied objectives and locales. DLC is also properly integrated into playlists and comes up often in my sessions, and thats a huge improvement.
Its a fucking great game, and better than I had hoped it would be honestly, but its definitely 343's beast now. Still Halo, but different.