Q. Which is best KTM DUKE 200 or Honda CBR 250?

Both are good but are from different segment. One full faired and the other a naked bike, engine capacity was also different but produces nearly the same power output and fuel efficiency. The honda's engine was more refined then the ktm's. Cbr uses a duel channel ABS with combined braking system while the duke200 uses just a single channel ABS with RLP. Moreover the CBR is a bit costlier.If it was me, I'd go for the CBR.

If you buy Duke it is better for future if any problem occures you can solve easilyBut in cbr it is very difficult to find some spares

What do you mean by BEST? Define what you want from your bike, then compare similar bikes. KTM and CBR are very different purpose bikes.