Which US server to transfer to?
Why not just transfer to the server your friend is on ?
or you won’t be able to WvW with your friend.
I would investigate what is important to you. If you mostly like PvE and LS and stuff you probably can’t go wrong – because you can always guest if you need something specific.
If you’re interested in WvWvW you might want to look more closely at the pros and cons of various servers. While being on an “always winning” server might sound great, it also means it’s harder to get INTO WvWvW in the first place – longer wait times. It depends on how/why/what you want to play. Wanting to be able to play that with a friend or not is also a big factor.
The most populated servers are Blackgate and Jade Quarry, unless you enjoy overflows for popular meta events and really really long queues for WvW, I’d suggest to avoid them, though it depends what time of the day you will be active.