My choice was a newer version of my existing phone, the Oneplus 5T. Initial observations are very good - nicely made, the rear-mounted fingerprint sensor which I was worried about is mostly in the right place when I need it, the battery life is dramatically better than the OP3, due I presume to the improved CPU technology (low power cores), as well as the fact the battery is newer and slightly larger capacity. It also has a HEADPHONE JACK and performs very well indeed. The speaker, which I use often, is louder too.
I paid £330 for a phone from eBay that was a few months old, and expect to get around £120 for my OP3, making my upgrade cost about £210.