What’s the best thing about Switzerland?

Q: What’s the best thing about Switzerland?
A: I don't know, but the flag is a big plus.

This is a pun. The flag of Switzerland is red with a white cross (which could be said to look like an addition sign.) So, in other words the flag could be said to have a large plus sign on it. The phrase "X is a plus" is a colloquial american English phrase that means "X is a thing that is positive or desirable".

So, the joke lies in the the fact that the person is listing the flag as a positive thing about Switzerland when I doubt that anyone really would consider having a flag with a cross on it to be that amazing.

Finally, there is an element of required knowledge to get this joke - if you do not know what the flag of Switzerland looks like, this joke would not make any sense.