I remember facing appropriate challenges with numerous Gym Leaders, and having a really rough time with Marshall in particular in Black Version. I was in high school and had been playing Pokémon for about four generations by that time, so while I wasn't as "good" as I am now, I feel like I had enough experience with the series that I shouldn't have struggled if it was easy. So for BW, I'd rate them a 9 or 10. Not that they're super awfully difficult compared to other, non-Pokémon games, but I feel like I was challenged more in BW than in any other Pokémon (main-series) game overall (comparatively, I was unable to defeat Cynthia in Pearl Version for a long time, but the rest of the game didn't pose much challenge for me, and I felt Crystal had a lot of challenging moments, but maybe that's just because I started with Chikorita...).
Alternatively, I managed to get through Black Version 2 without much difficulty, and while I had a full team of six, I really only used five. They still felt appropriate, though, so I'd rate them around 6 or 7; not the hardest but certainly not the easiest the series has to offer. Although, I feel this score would be much different if I was able to select Challenge Mode from the get-go, rather than wait until I beat the game...
Granted, this is going by what I remember. It's been a while since I've played any of the games I mentioned in this post...