Halo Reach Ridgeline MapHalo Reach definitely looks better. It was due to the Artstyle. Bungie did a great job with Reach. They really maximised the tech used on the 360. And not only that but the maps you play on feel like they are in the Universe. Halo 4 and 5 maps don't feel that way. They really do feel like a simulated version, and they come across as soulless as a result.
This. Bungie's maps felt lived in, 343i's feel artificial, which takes away from the enjoyment, despite how much I like the gameplay on 343i's maps. I love Halo 5's Forge, but it needs a bit of a paint job to make the map pieces more organic with different options for skins and such. Of course that probably wouldn't be possible until Halo 6.
As for Reach, I think that looked better because it was gritty. Like what Richnj said, the world didn't feel artificial, it felt real. It's so much easier to suspend your disbelief and immerse yourself in Reach, whereas the bright matte colours and glowing Spartans of Halo 4/5 really take that away from the player.