The Dreamcast shouldn't be included in these 5th gen comparisons, its a substantial step up from 5th gen and Sega already has the Saturn to represent it for that generation. Its tough finding screenshots/videos of games that haven't been enhanced using emulator features. I will also stick to 3D games without pre-rendered backgrounds or 2D games, as those look better than most of the other choices. Here are some that I thought looked great:
Ride Racer Type 4 - An absolute stunner on the PS1. The tracks are quite detailed, cars looked great, and the lighting in tunnels and day/night transitions was really impressive, and its all ran at a smooth 60 fps. They were squeezing everything the PS1 had for this game, and its a visual feat.
Crash Team Racing - Super colorful, with loads of details on the tracks, and a lot more going on in the backgrounds than Mario Kart 64.
Mega Man Legends 2 - I fucking love the art style of this game, and its quite the looker for the system. The draw distance is also pretty good for games of this era.
Bulk Slash - The Saturn isn't known for its 3D strengths, but this game looks really good IMO. The pop in is very noticeable when in flight mode, but the game still looks quite good, and runs really well.
Banjo Tooie - I may like the first game more, but I think the second is a bit of a step up over the first visually. The areas in the game were quite large too, which was impressive for the time, and without even relying on the ram pack.
Diddy Kong Racing - The best racing game on the system IMO, it looks much better than MK64 IMO, and has much more going on with its tracks.
Legend of Zelda: Majora's Mask - I could also put Ocarina of Time here since they both have a similar look to them, but I think Majora's has the edge. Clock Town and the dungeons were really large and looked great.