Atari Jaguar. I picked it up on eBay back in 2012. I was on a big retro kick at the time. Was it worth it? In terms of the game library, not really. Tempest 2000 remains one of the best games of the 5th gen, and Alien vs Predator is a lot of fun, but there's not a whole lot after those two. I do think I could easily turn a profit, though, if I sold it today.

Next, I think, is 3DO (FZ-10 model). The Need for Speed and Road Rash are fun, but a lot of the games in the 3DO library are available elsewhere. Not worth it.

After that, Dreamcast. Absolutely worth it. I do think its greatness tends to be slightly exaggerated in gaming circles, but it's a special system with some amazingly stylish games. Honestly, I think it's worth it for SoulCalibur alone.

Don't make me list every 9/10 DC game! It has gotten a ton of support from Indies over the last 25 years and a lot of games that got mediocre receptions have aged like fine wine.

I'll include PS Vita because the total could be as high as around 16 million units, but it also could be in the low 13 or so range slightly below Wii U.
Wii U I do not regret purchasing. I got for $250 brand new in the Deluxe Set and that was the promotion where for a time you could register your copy of Mario Kart 8 and get a free game (I picked NSMBU).
Wii U was a cheap system to get games for over the years. I got some on clearance and the online in general was free.
My memories with Super Smash Bros. for Wii U and several other games make it more than worth it.
PS Vita I get the OLED model used 5 years ago. I don't regret it as I have had fun with some of the true exclusives to Vita but also multiplats or interesting versions. I have also used Remote Play from PS4 sometimes.
I don't think I've ever regretted a hardware purchase. Even Switch 2 at least I got the Mario Kart bundle which was a good deal. And I have only not put a lot of hours in because of I have a PS5 but some older systems too. if Switch 2 was one of my few platforms (especially of modern ones) I probably would've used it for 500 or more hours by now.

Can't find any concrete numbers for the PCE/Turbo Duo, but of the rest Sega CD sold the lowest.

The Sega Nomad: a full-blown Sega Genesis as a portable handheld. I would literally lease this thing to my classmates for $3 per recess. I was making $30-45 a week in addition to being able to play my favourite games in the car.

I bet my teachers thought I was going to become a drug dealer one day.

Xbox og,totally worth it

Commodore 64 sold how much? PCE did at least 6 million in Japan. 

Edit: Looks like it did 10 million worldwide between the Express, Duo, Turbo and PCE stock. 

C64 sold between 12 and 17 million units:
https://en.wikipedia.org/wiki/Commodore_64

My first system was a C128, which sold ~2.5 million units. Totally worth it!
https://en.wikipedia.org/wiki/Commodore_128

It wasn't THAT much more expensive than a C64 and I loved the new design of the computer case and the new floppy disc drive.
(They also used the new design for the C64C aka C64 II). Other advantages of the C128 were a full keyboard (Num-Pad and 12 F-Keys instead of 4), Commodore Basic 7.0 instead of 2.0 with a lot of additional commands and a CP/M mode.

So Basic programming was much more fun in the C128-mode. And the C64-mode was 99.9% compatible to the over 5,000 commercial C64 games.

Wii U.  I think it was just barely worth it, because I liked it better than the other disappointing systems that were available at the time.  PS4 was a big disappointment for me, and so was the 3DS.  Wii U was also a disappointment, but it had a few fun games.  Unfortunately, the Switch made the Wii U obsolete, and my gamepad keeps having batteries that die, so it's not worth using in the long term.  As I said, the Wii U was just barely worth it.

Neo Geo AES for me, which sold around 410,000 according to Wikipedia.

It was worth it just for the novelty of that insane 16-bit arcade graphics, some of the pixel art was just sublime.