Advice for a relative newb?

I've searched through the sub and it's a lot of fun reading your posts, but I'm particularly trying to find advice on best monkeys and combinations, which upgrades are best, etc. I have beaten the easy levels and earned most of the badges, but I struggle with the intermediate levels.

In particular I have trouble with any map with more than one path for the bloons to follow.

Your helpful advice would be appreciated!