I think that best offs are really ideal for people who either kind of like a band or for someone that wants to try them out. The thing is, Joy Division are one of the best bands of all time. I could have the best of David Bowie, but I don't - I have all his albums and have enjoyed the music much more for it. There is no substitute for an album.
There is also a very natural flow to albums like Unknown Pleasures which you will simply miss out on with a best of. The inclusion of the Peel sessions is good, but realistically that's only for a true Joy Division fan and they will probably already have them.
My advice is to get Unknown Pleasures first if you're new to Joy Division - it is one of the most startlingly accomplished albums I have ever heard. From there, get Closer and if you're loving what you're hearing seek out such amazing tracks as Transmission, Atmosphere, Love Will Tear Us Apart and Dead Souls. Joy Division have an incredibly strong catalogue of songs. The fact that they kept some of these tracks/singles off their albums might hint that they weren't good enough, but the truth is that this band was so good that these singles matched if not superseced some of the album tracks!
I'll finish by adding that if you do love Joy Division and want to hear some bands that have similar styles, try out Magazine, Echo & The Bunnymen (particularly Heaven Up Here) and early The Cure.