xbox 360 slim

The PS3 Slim lost backwards compatibility.  That's a tough argument for a "best" slim when losing a feature like that, IMO.  At least from any kind of an objective viewpoint.  Why do you choose the PS3 Slim?

PS2 Slim. Crazy slim and sleek!

Ps2 slim. No one has matched that size reduction and overall slimness along with keeping everything

Xbox One S

I know it's not a "console" but I'd say DS Lite. The original DS was an absolutely horrid looking thing, and the buttons were awful. The DS Lite was such a huge improvement. Looked a million times better, and the buttons were nice and squishy.

If that doesn't count, I'd probably go with the PS2 slim because of how much smaller they got it to be.

I'd say the PS3 slim was more reliable, quieter, ran cooler, and smaller with a nice design while still playing Blu-rays and having a front loading drive.  Besides BC it was a better PS3 .  Also they kept the power supply internal in the PS3 slim, unlike the PS2 slim which now had a power brick.

I have the One S tied with the PSOne

I'm gonna have to second this, if handhelds count.

As for home consoles, 360 slim seems like an easy choice, as it finally resolved the catastrophic RRoD problem.

PS2 Slim 9000 series for the fact that they reduced the size of the machine so dramatically AND also incorporated an internal power supply(where the PS2 Slim 7000 needed an external power supply). They also added a built in ethernet port which the original phat PS2 did not have. All that was omitted from the original PS2 was the firewire port which was seldom used anyway.

Also the Xbox One S for its improved functionality of an included UHD blu ray player, HDR and slightly up-clocked GPU speed. Just a better looking model over the original in everyway.