I'm going to date my best friend who dated my friend
What makes you want to date your best friend?
It's because she's the only one I really like who's single
I see your point, but it's not nice to date a friend's ex. This could really hurt your friend's feelings.
There's no going back but it doesn't really matter to me
Dating a friend's ex partner will damage your friendship. If you choose to be with this person, you should talk to your friend about it. It's best they find out from you than from someone else.