Knowsley doesn't open all year round, so depending on when you're coming that option could be taken away from you.
I agree the Polar Bears at YWP in snow would be fab, but don't build your hopes up, snow is not that common, you'd need to be fortunate. What a prospect if you were to be though. Trentham would be good in the snow I'm sure; its on my list to do if I get chance over the winter, but as good as it is a monkey exhibit the place is a one trick pony and you won't spend long there. Trentham is close to Peak Wildlife Park but I think that might be a challenge to get to in the snow!
I would do YWP, Blackpool and Chester out of the collections you've listed, and for maximum indoor time, Chester wins.