What system was the biggest improvement over its predecessor (in overall quality, not graphics) and why?

[Leynos]

Saturn to Dreamcast. Saturn as much as I love it thanks to the huge and amazing Japanese library. In the west it was lackluster library (some greats) and the US launched with a different controller than Japan. It sucked. System had a big clunky memory card slot. Also used for ram expansion.  Saturn itself was kinda big and ugly. Dreamcast was sleek. Futuristic looking. Cool VMU memory cards. A slick UI for the time. It just worked. Online being built in. Analog sticks out of the box. A demo disc. Web browser in the box. 

N64 to Gamecube. While personally not a huge fan of the Gamecube controller. It was a huge step up from the N64 pad. The neat trigger buttons. Gamecube looked neat. Oozed personality. Simple but sleek UI with the Famicom Disk system music played in slow motion. Not as cool as the PS2 memory card icons but Gamecubes were neat. Before it set in how limited the discs were. The fact the games were on discs was cool. How small they were was neat. GBA Player was great! 

Xbox to Xbox 360. Similar to Saturn to Dreamcast. Everything about it went sleek and futuristic experience. 360 took the media features of Xbox and expanded on them. Win Media Center. Connect MP3 player. The blades was the coolest UI console idea then and now. For many Achievment points was an interesting and novel concept then. User rankings. Similar to Dreamcast you got a free puzzle game right away. Sleek online features. 

Gamecube to Wii. Wii was so unique. The pointer just worked. The channel concept was novel and fun. I spent a lot of time in the news ones to see the cat. Mii Votes channel. You could have fun with the channels before you play a Gamecube or Wii game. Not to mention how sleek and cool Wii looked esp with the blue pulsing light. 

Wii U to Switch. Wii U was a half step to the Switch concept. Wii U was clunky in looks and in function. The UI ran slower than a 3 legged turtle crawling in a puddle of molasses. Switch cut the fat of that UI. Back to something bare bones simple but was fast, Clean. The system was the finalized idea Wii U tried to achieve. Switch was simple but elegant in function hardware and software.