I think the Nexus 5 has had the most attention historically, and may work best today. They have officially stated that the PinePhone is the future though - between the fact that it is easily available (to developers now, general availability expected in a couple months), and there is enough information that we think it is possible to make everything work it will soon be the best.

Nothing is currently usable for everything. PinePhone will soon be usable for phone calls, something nothing else can claim.