I have thought every Halo game thus far has been graphically impressive in one way or another.
Textures, scale and effects in Halo CE were amazing at the time. I remember coming from the PS2 to Halo CE my mind was blown.
Halo 2 had amazing lighting and cinematics, despite a worse LoD system and the lack of detail textures from Halo CE.
Halo 3 is a graphical powerhouse in everything other than resolution. The HDR lighting is incredible, the detail textures were back in full force, the water is unmatched to this day, the scale of the battles was insane, and there were so many tiny details and touches that I still notice new things to this day.
Halo ODST was largely the same as Halo 3 (obviously) but with some more immersive details like first person cinematics that I loved.
Halo Reach maintained and improved texture detail, geometry, facial models and animation, cinematics, particle effects, motion blur, increased resolution and AA, brought back the great lighting from plasma projectiles that Halo had been missing since CE, and had an excellent attention to detail graphically. Though some did not enjoy the somewhat "dull or drab" art style and tone, it was graphically a very impressive game.
Halo CEA was a stunning looking game with most of the graphical prowess of Halo 3 and Reach, all while running the old engine under the hood, supporting 3D, etc. Really, being able to switch back and forth on the fly is one of the best ideas they could have implemented to Halo CE, very technically impressive and I used the feature constantly.
Halo 4 has brought us a new lighting engine that is very much improved in most (but not all) areas , a high level of texture detail (while somewhat lower than H3/Reach), vastly improved character models and facial animation (no contest), much improved native 720p and AA solution, rock steady framerate, improved water over Reach (still not H3 level) along with much more of the immersive techniques like the HuD, first person scenes, the inside of MC's helmet reacting to light, etc. Still a huge scale for an FPS game, yet not quite reaching the heights of scale that Halo 3 or Reach did.
This game is beautiful, is it beautiful enough to warrant the Bungie hate? Hell no, all Bungies efforts have been great, and while 343i looks amazing, and is a step up, it was not without sacrifice, they simply allocated resources for a more impressive overall image, while toning down things that only people like me really notice. Smart move when you are trying to get milage out of an 8 year old console.
I think 343i did an amazing job, especially for their first effort as a team. Once Halo 5 is shown, as I have said before, our senses will be obliterated by how amazing it will look.