Honestly, that isn't something that truly can be answered. Each person will have their own opinion to whether you should skip an episode/season or not.
In my case, I have watched almost everything and it takes time, so if you don't want to spend time on general filler episodes, then skip to the big battles, like the gym battles. Most people want to see these rather than to watch the whole season.
If you want to skip a large amount of episodes, the you could skip a whole region. In Black & White, they basically did a restart so that the newer audience could enjoy it better. So that's a pretty good season to start watching. If not, then watch the few last episodes in Unova so that you know his reason for going to Kalos. (Kudos to "A Geeky Jawa" for pointing this out.) If not, then you could alternatively skip directly to Sun & Moon, although I can't say anything from personal experience because I haven't started to watch Sun & Moon.
So in conclusion, you could basically skip everything. But every time you skip an episode, you lose an episode worth of enjoyment.
Not sure if I helped or not, but here it is!
