Fastest SSD for MacBook Pro?

Hello, I recently just got a MacBook Pro 2015, and this is the first Mac I've ever had.

I was wondering what the fastest SSD is that is compatible with my MacBook (Model 11,4) would be. I've tried looking around and most of the posts I've seen are old (several years) or the SSD's are rather expensive.

What's the best bang-for-buck performance from an SSD that I can get?