Hardest? BR.
But I'm not sure if harder = better. Sometimes they are made harder by expecting you to have a bunch of background info, this is "hard" which doesn't help.
If you really wanted to be good at bio, it is thinking in pictures. This is why upper division courses are great for bio. You start to build a great mental library of biological pictures. Great textbooks or videos or whatever can help you here too.
I enjoy BR but sometimes don't. Their bio passages will probably be the best, but don't go in there trying to ace them as some questions may be silly. EK is just going to familiarize you with everything and prime you for reading science stuff/knowing the material.
EK is probably more focused on what you need but less passage based. EK is generally considered easier also.