BEST
I have very fond memories of the first Xbox. Still the only console I've picked up at launch. It was very expensive in my country (the equivalent of 760 USD today) and they slashed the price by 150 USD after only two months giving us who bought it for 500 USD an extra controller and two free games (not many to choose from tbh). I was a student at the time but once I played Halo it was all worth it. Those jaw dropping moments that game had is something we just don't see anymore. I remember racing through my own town of Stockholm playing my own music on that Swedish radio station in PGR2. It all felt magic, really like the next generation of gaming.
I also remember Xbox 360 very fondly, despite the hardware issues. Halo 3, Condemned, PGR3, Alan Wake and tons of other games made it an amazing console. After 2010 it all went downhill for Xbox, and I bought the PS4 before I finally got the Xbox One, which still ended up as my main platform that gen (One S and One X were really good).
WORST
When everything was about Kinect. Seriously, look at the E3 shows after 2010. I know Nintendo started it with the Wii, and the great success of the KInect sort of made this choice for Microsoft, but as a core gamer it felt shit. And then the Xbox One reveal, obviously. In May 2013 I was so hyped for the next generation of Xbox. You all know how that went. I've seen that reveal over and over again, trying to understand how they could've thought this was a good way to reveal a console. And I still don't get it. I just bought the Xbox One Kinect to try it out since I got the console when they had unbundled it, and I have to say I can sort of see some of the appeal. But not enough to warrant the extra 100 USD and the whole console experience being built around that thing.
FUTURE
Xbox as a big hardware platform is history, and Xbox gaming is turning into Microsoft Gaming. I think Microsoft, at least for the past ten+ years, have seen hardware as a necessary evil. Now they can finally start to move away from something that's basically never made them any money, concentrate on games and services and make more money than ever. As someone who loves consoles and have always liked the Xbox hardware, I'm disappointed it's coming to an end. But we've seen the writing on the wall for quite some time now so it's not a surprise.