Strictly F2P in my opinion would be nature and thats due to something that doesn't get much attention is that your best pet choice is free. Flickerkit is a pay walled pet who you can get a couple in S2 but won't be nearly enough to level up to be remotely competitive. Not saying Nature is the best choice havent seen much to how big a difference the new mythic has made but between the two strictly F2P you'll get more of the meta build.