This is the dawning of the age of Aquarius! Gamers I welcome you to your new home! Aquarius! Aquarius! Aquarius!   ...it's only a console.  SHH!

...on second thought let's not sell Aquarius, it's a silly system.

Nokia N-Gage. 3 Million sold according to Wikipedia. But I loved that thing. It could do lots of stuff that smartphones were praised for many years later. It did MP3, Internet browsing and calls long long before Apple introduced the first iPhone and was so proud of combining these three things in one device.

It also had some great games. Playing Tony Hawk on the go back then was a friggin' miracle. And Pathway to Glory was one of the greatest strategy games ever made.

GameCube. Loved that purple box.

Metroid Prime, Fzero, RE4, Paper Mario TYD, Skies of Arcadia Legends...so many gems.

Lol I guess that is the best answer you can give 

There were other colors for GC but that didn't help sales one bit.  PS2 wiped the floor that generation but GC had Double Dash and Melee.  Better local multiplayer games for sure.

Not sure if already posted, but I had the Gameboy Micro (only around 2.42 million sold according to Wikipedia).

Still own a Dreamcast. Can't believe that console only ever sold 10 million, definitely the most underrated console with some stellar titles - though I suppose Sega had marred their image beyond repair via marketing and stretched themselves a bit too thin with all the hardware and accessories. 

Owned (and sold back) the Wii U. Was never really a big fan and it's probably my least favorite Nintendo console - though I did somewhat enjoy Mario Maker, NES Remix, (some parts of) Nintendoland, Xenoblade X, Yoshi. Probably didn't deserve only 14 million sales but definitely Nintendo's weakest, most gimmicky effort.

GameCube - as you've probably gathered from my username/avatar I owned a Cube, though sold it back as my Wii now handles the few Cube software I've hung onto via backward compatibility. Definitely underrated, not the greatest Nintendo first party software on that console for me but some great 2nd and 3rd party offerings like Tales of Symphonia, Eternal Darkness, REmake, Time Splitters, Metroid Prime, F-Zero, PSO Ep. 1 & 2, etc.. Plus it's home to what I consider to still be the best version of Smash Bros. 

For me, it would be the ColecoVision or Atari 7800. VGChartz lists the 7800 as 4.3m sold. I've never seen a concrete lifetime sales # for ColecoVision, but I've seen it estimated anywhere between 2m-6m. I believe this is because the last reported figure was well before the video game crash of '84 in NA and the console was still selling for quite a while after that report was made.

True words. As launch DC owner, The DC had better games for pretty much 2 years. The PS2 had a dysmal launch with few good titles, but its hype was massive.

It depends what is counted as a system. As far as kind of well known video game consoles go, maybe the Atari 7800.

However, some less clear contenders are the Barcode Battler II and the Socrates edutainment console by VTech. No idea what kind of sales they had, but it can't have been much, though I don't know if the Barcode Battler II is distinct from other models and I know they had popularity in Japan, so sales may have surpassed the 7800. Strangely, the PocketStation is no contender here, having passed 5 million sales.