My aging Galaxy S4 has finally gotten to the point that I want to replace it. Which Android phone should I get?

My basic philosophy is not to buy the newest version of anything unless the performance improvements are hard to beat. I use my phone almost exclusively for email, phone calls, web browsing, and navigation, along with some occasional document editing, transit apps (Lyft/Uber/Zipcar), and podcasts. Very boring, and it means things like camera and display quality are of pretty low priority. I don't really want an unworkably large screen, but I would definitely appreciate something relatively durable and with a good battery - my typical refresh cycle is 3-4 years (a replaceable battery and a microSD slot will be a big plus). Price is not really an issue other than the fact that I don't want to pay $800 for something that will cost $500 in a year, and I see no reason to spend $500 on something when $300 will work just as well.

Suggestions? I know some of you really care about this stuff and will have an opinion. Remember, though, that I'm looking for functionality, not fashion.