-/
/
|(::)(::)(::)|
|(::)(::)(::)|
|(::)(::)(::)|
|(::)(::)(::)|
[[[[[[[[]]]]]]]
Well, Halo 2 I would have to say, mainly because it is challenging, it teaches you to take it slow, be patient.
Halo CE is, well... It is the first game of the series, and so is not quite as fleshed out as Halo 2. Halo 2 didn't have the best story.. But its game-play was amazing.
Halo 3 was too easy, you could tank through it just as well as you could in Normal.
Halo 3: ODST was one that was properly fun, but that's a personal opinion, not many people seem to like it and so I don't recommend it in any case...
Halo Reach was a breeze, they made it too easy. The only thing that ever posed a problem was elites, and even then, with one well placed Plasma Pistol overcharge followed by a head-shot, that problem was gone.