the nes, now that i think about it. i never had one, and by the time i got it, it was just a few months away from getting a n64. either that or the original xbox.
My original Xbox. I bought it right at the end of its life and then ended up getting a 360 like two months later. It's been in its box on top of my wardrobe ever since.
I never really liked the DS. A huge, but very meh selection of games, with the best ones feeling just average to me. It couldn't hit the same highs as its contemporary the PSP.
The GBC was basically a pokemon machine and not much else.
The last gen also comes as a close contest for least played. I have hardly more than 5 games on each console (WiiPS360), it just wasn't very good and the PC/PSP took over me.
My PS3, and lately my 360. Literally only bought it to use as a BluRay player and to play a handful of exclusives like Uncharted, The Last of Us, and Journey. But between my PC, 3DS, and Wii U, I just never really touch them any more. 360 went from most played to not played this past year.
My least played console is my PS3, because I'm scared to play it. It's my second 60gig, my first one YLOD'ed on me, and the replacement refurb I bought from GameStop has a very, very loud fan. I do plan on buying one of those slick blue super slim PS3's in a few months to catch up on some of the games I've been sitting on (Tales of Xillia, Tales of Symphonia),.
My PSP and my DS Lite. When I got my handhelds back in 2006, it was amazing at the time how far along handheld gaming had come and I was playing a ton of it, but by 2009, I was barely playing handhelds other than the occasional game here and there. Console and PC gaming pretty much overshadowed it for me and it's been like that since. I go back here and there, but nowhere near as much as the consoles/PC.
As a result, I still have yet to upgrade to a Vita or a 3DS.
N64 back then as I only ever bought like 5 games for it. Those games were expensive as hell for a kid with no salary. Mostly stuck buying PS1 games as they were cheaper and had the overall better library.
Currently, my PS3 and Wii U are pretty much on equal footing.
Of all time, I'd say the PSP. I bought maybe 4 games for it, and only really finished two or so. For whatever reason the catalog of games never quite clicked with me.
Vita. Messed around with some system settings and Netflix, used it to take pictures until I got a phone, played some demos, 10 minutes of that MotorStorm RC game (because it was free), and Gravity Rush (which I bought it for). Stopped using it over a year ago. It's literally collecting dust right now. I don't even care anymore, the VIta is dead.
PS2 since I've a PS3
PS3 since I've a PS4
Wii is unplugged since 1year+
my PSP is untoched, since I have a PS Vita.
3DS only exists for Pokémon, at least for me, my Wife is currently playing Bravely Default
It changes in bursts for me. My WiiU was collecting dust for the longest time even though I have a bunch of unfinished games for it. When the GBA games came out last weekend i bought those and not only spent all weekend playing those but also went back to the games I hadn't finished. I can't wait for the weekend to roll around so I can play some more Advance Wars, Wind Waker HD, Wonderful 101, ZombiU and possibly Monster Hunter*.
Similarly, I was all about the Vita until I bought Tales of the Abyss on my 3DS and now for the past few weeks I haven't really touch it. I'm sure once I get bored of my Wii U and/or 3DS I'll jump back to my PS3/Vita. If we're talking about pure hours clocked I would probably say that out of everything Vita has the lowest but it probably has given among the most fun per hour out of all my gaming devices.
*Holy freaking crap. I can't beat the stupid punk Qurupeco and I don't even know why. In Monster Hunter Freedom Unite I could school that sucker no problem with a light bowgun but now that I'm trying to run with longsword I can't beat it for the life of me.
Not counting portables (though the Vita may be a contender for most played if I did) and the Wii U (lent to kid brother until he starts college/Army), I'd have to say currently the 360.
Haven't touched it since Halo 4 except to build some stuff for my wife's young sisters on Minecraft and to catch my wife up on Bioshock Infinite after the 360 (or the game) kept wiping the save data due to not being connected to Live. I still have some backlog on the system concerning Red Dead Redemption and Arkham City/Origin, but those are very low priority in the backlog.
Hell, I play my Gamecube more seasonally nowadays.
Edit: Lifetime, in consideration of newly released hardware, would probably be the PS3 by a slim margin.
360 before I sold it (other than Halo there wasn't much that I couldn't just get on PC :/)
These days it's a draw between my 3DS and Vita. Played Animal Crossing a ton but haven't touched it since September or so. Will be using my Vita more for remote play though.
Wii U. I initially got it to play The Wonderful 101 and Super Mario 3D World, which I did, and after that I ended up playing through The Wind Waker HD but after that I haven't done anything with it since. The last time I turned it on was this past December.