From campaign point of view, I prefer ODST to HALO 3, which translates to HALO: CE, HALO 2, ODST, HALO Wars and HALO 3.
Bungie altered/changed too many visual aspects in HALO 3 and didn't felt like a squeal at all. Not to mention the acting was poor at times and cheesy, (which also applies to ODST) and had no complexion or depth. The engine and excessive blooming is whole another story. I still don't understand why developers apply too much HDR+Bloom (maybe to distrace the players from flaws?)
TBH, I can go on forever.
I just pretend HALO 3 doesn't exist and I made my own ending to fill the void. I do however play the multiplayer as the visuals are closely linked to HALO Wars, H1 and H2.
PPS: I spend more time playing HALO than watching movies or playing other games. Hell, I've even played HALO 2 campaign more than 25 times and play it twice every year. But HALO 3 really did disappointed me. I had such high hopes. Hopefully Reach will fully restore my faith in Bungie. If not, well I can always play HALO 1 and 2.
[Edited on 04.10.2010 7:51 PM PDT]