Just for clarity's sake, more successful doesn't necessarily mean "which do you like more", like if it was just "which is the better console" I would have written that instead.

For instance, I didn't much like the DS or PS2, but I'd obviously consider them extremely successful.

Then its a question you already should know the answer to.

Xbox series.

Made more money.

Close the topic

There's more interpretations of success than just raw revenue though; there's also broader strategic success, mindshare, etc.

OG Xbox, it established what would become huge IPs, not just for Microsoft but the entire industry. It introduced dedicated OS and hardware storage to the console market and even had online; it laid the entire groundwork for everything the 360 became (which enabled it to go toe-to-toe with Playstation in the following generation). It competed against two titans, and even managed to beat one of them, from literally no pedigree and/or experience in the console space.

Sure, we can stare at the monetary losses, and the low lifetime sales, but that's not what it was about in hindsight. Without the OG Xbox, there would be no other Xbox, including the Series. 

If we look at the Series for comparison - all it has managed to do is lose MS huge swaths of ground in the market, it led them into a full-on identity crisis and sits on the hem of the worst financial results MS have shown in total since the economic collapse of 2008. Heck; this very site is full of discussions on whether Xbox as a brand is dead, or at least dying, and this is mostly due to the poor showing of the Series. This has been a disastrous generation for MS, with the Series signalling dark clouds and customers fleeing en masse, whereas the OG Xbox signaled a company and brand aiming high and positioning themselves via technological and feature-driven innovations and a completely new look at what consoles could be (and indeed, became after). 

The Series feels like it was sent out to die, more or less, there has been no oomph and fanfare around it at all, and the software showing has been the weakest in Xbox history (in spite of all those studios acquired). Just look at the numbers of studios and developer lay-off in the past few years, of course other companies have done the same in the same period, but I don't think anyone comes close to MS in sheer numbers and impact. 

To dub the Series a "successful" console with all above parameters in mind is quite the stretch. By economic metrics and argument alone, the Wii U is a success story since it was profitable, I doubt we would find many who would stand behind such a statement. 

The OG Xbox made Microsoft relevant in the console space. Xbox Series made them irrelevant.

It really is easy to answer. Answering Xbox Series is just crazy talk. Sure you can look at just revenue number and number of consoles sold, but that’s just ignoring all the other circumstances. Besides Series X has had almost 50 % longer life than the original Xbox, so the fact that it has only managed to sell about 50 % more, really isn’t that impressive. When the original Xbox launched, Microsoft had to build a brand, attract consumers, convince users of the extremely successful Playstation that they could get a better experience elsewhere.
When the Series X launched they had market presence and a strong brand already, and had acquired a lot of new studios. Still they messed up so much that barely anybody care about their ecosystem anymore.

Goggles

Also, as you can see in the thread, people disagree with you in many ways.

Sales or profit are not the only metric people are considering as success has many forms.

XB left MS a good path ahead, stablished successful franchises, has contributed to online gaming becoming a mainstream thing on consoles. It also managed to beat Nintendo in sales, so even that metric is not 100% in favor of the XBS in a way.

XB is far more important to gaming than the XBS, and to me that can make it a more successful console, as it managed to leave an impact in the industry the XBS didn't.

In terms of building brand its original XBOX
In term of revenue its Series but that is because you didn have million ways to milk people when original Xbox was released.
Original Xbox build the brand and Series completely destroyed it

Xbox 1 gained the respect of the gaming community and then some. Xbox 4 did not. And not only that, it pretty much put the brand out of business. So the first Xbox wins, easily in my opinion.

Xbox Series X/S made more revenue and surely higher profits.
Xbox established the brand with its hardware, Xbox Live, Halo, Forza, and so on. And it led to Xbox 360, the most popular Xbox console.
Xbox was a net positive for the brand; Xbox Series seems to be a net negative.
I voted Xbox.

I owned neither, og Xbox managed to beat Nintendo in their very first generation, Xbox Series managed to kill Xbox as a console. Now all you're gonna own is Windows PCs with an X stamped on the back.