Is new best?

You need to compare the two specifications with the requirement. I know little about the 250D and DX0 do not appear to have tested it. One of the critical requirements for me is dynamic range which hardly gets a mention in advertising. I know it is excellent for the 800D (around 14 stops) and my current Canon 5D4 is similar. There are many other parameters I would want to compare before I decide which was BEST for me. This would not necessarily be the best choice for someone else.
Dave
Dave

looking at https://www.apotelyt.com/compare-camera/canon-250d-vs-canon-800d
The 250D is smaller and lighter and takes more shots per battery charge, they both have the same sensor but the 250d has a later advanced image processing engine.
Arguments in favor of the Canon EOS 250D:
Better jpgs: Has a more modern image processing engine (DIGIC 8 vs DIGIC 7).
Better video: Provides higher definition movie capture (4K/25p vs 1080/60p).
Larger viewfinder image: Features a viewfinder with a higher magnification (0.54x vs 0.51x).
Easier time-lapse photography: Has an intervalometer built-in for low frequency shooting.
More compact: Is smaller (122x93mm vs 131x100mm) and thus needs less room in the bag.
Less heavy: Is lighter (by 83g or 16 percent) and hence easier to carry around.
Longer lasting: Can take more shots (1070 versus 600) on a single battery charge.
More affordable: Was introduced into a lower priced segment (20 percent cheaper at launch).
More modern: Reflects 2 years and 1 month of technical progress since the 800D launch.
Advantages of the Canon EOS 800D:
Faster burst: Shoots at higher frequency (6 vs 5 flaps/sec) to capture the decisive moment.
Easier device pairing: Supports NFC for fast wireless image transfer over short distances.
More heavily discounted: Has been around for much longer (launched in February 2017).
The 250D is smaller and lighter and takes more shots per battery charge, they both have the same sensor but the 250d has a later advanced image processing engine.
Arguments in favor of the Canon EOS 250D:
Better jpgs: Has a more modern image processing engine (DIGIC 8 vs DIGIC 7).
Better video: Provides higher definition movie capture (4K/25p vs 1080/60p).
Larger viewfinder image: Features a viewfinder with a higher magnification (0.54x vs 0.51x).
Easier time-lapse photography: Has an intervalometer built-in for low frequency shooting.
More compact: Is smaller (122x93mm vs 131x100mm) and thus needs less room in the bag.
Less heavy: Is lighter (by 83g or 16 percent) and hence easier to carry around.
Longer lasting: Can take more shots (1070 versus 600) on a single battery charge.
More affordable: Was introduced into a lower priced segment (20 percent cheaper at launch).
More modern: Reflects 2 years and 1 month of technical progress since the 800D launch.
Advantages of the Canon EOS 800D:
Faster burst: Shoots at higher frequency (6 vs 5 flaps/sec) to capture the decisive moment.
Easier device pairing: Supports NFC for fast wireless image transfer over short distances.
More heavily discounted: Has been around for much longer (launched in February 2017).

I agree with comments in that 'Best' is subject to the criteria requirements. To me a good camera should be easy to use, intuitive and have very good AF. If AF is pants then the rest of features are useless to me.
800D v 250D,...hmmm well I might opt for 250D as it has newer processor. As canons processors are behind sony, nikon, fujifilm you all the hp you can get in a canon camera !
800D v 250D,...hmmm well I might opt for 250D as it has newer processor. As canons processors are behind sony, nikon, fujifilm you all the hp you can get in a canon camera !