Nintendo: The Switch. Not only does it combine the strengths of Nintendo's handheld and console divisions, the whole system just shines. Has a great library, especially of RPGs. I haven't had this much fun in gaming in years. Spent more time playing BotW than any other Zelda game. We just got another fantastic Switch game in Fire Emblem: Three Houses. It's been a great retro system with a lot of arcade collections plus Nintendo's arcade library. And the ability to switch between TV and portable modes just by docking or undocking it is icing on the cake.
Sony: PS2. Much as I loved Gamecube, the PS2 just dominated overall. It was the leader of the incredible sixth generation of consoles. Had an eye-popping library for its time with a heathy mix of Japanese and western games. Sony just had the golden touch at the time, which they unfortunately squandered with the rough rollout of the PS3 that gave the 360 a big opening. That said, the PS4 has recaptured a lot of the PS2's glory after the huge stumbles of the PS3.
Microsoft: Xbox 360. Since I'm not a big Xbox guy, the 360 kind of wins by default since it was the only Xbox to date I've actually owned. Playing fighting games and even Halo 3 on Xbox Live was pretty revolutionary for its time, but the lack of Japanese games later in its life and the E74 error kind of killed the 360 for me. Still, I spent a lot of nights having a good time on 360.
Sega: The Saturn. It was an expensive failure that had a better library than it had any right to have. Panzer Dragoon Saga, Shining Force, Burning Rangers, Fighters Megamix, and even some obscure titles like Magic Knight Rayearth. Too bad Sega wouldn't bring over Sakura Wars or let Working Designs bring it over.