Source ProRes 422 Proxy / Edit / Export to ProRes 422 proxy?
Definitely proRes 422HQ. 

While you won't gain anything by encoding in a higher quality codec, what you will do is avoid losing more. 

Proxy codecs are just that: stand-ins for the real content, their job is not to faithfully encode the material, it's to be lightweight and easy to play, so you can swap in the full quality copy later. Whoever it was that shot Proxy, give them a whack upside the head to remind them not to do it again.

Export to proRes422 HQ as your master, if you need copies that don't take up so much space, say for delivery on the web, then transcode to h.264.