I want to love the Unihertz Atom, but sadly it just doesn't work for me.
I got it specifically to have a phone to only call, text and message. But those are the exact things that don't reliably function. It often didn't receive messages or even calls unless the app was opened and in the foreground.
I factory reset it, tried activating and deactivating Unihertz's own energy management app, tried the original versions of Signal/Threema/Dialer/Messenger as well as their FOSS forks/alternatives, tried it with and without Google Services installed...

But whenever I thought it works now, after lots of testing, it would then stop getting the notifications again after a few days, without me changing anything. So eventually I gave up.

If it had worked, it would be my favorite phone. But it didn't. Now it's rooted and I just play around with Android on it.
(By the way, if someone knows how to re-enter deleted IMEI numbers on a Mediatek phone from Linux to get rid of the red warning text overlay, please help!)

OnePlus 7T Pro 5G McLaren Edition.

I would still be using it if I hadn’t taken it into a pool with me and bricked it. 😢

I thought about this, and I'm not really sure. A lot of the phones I liked using are before 2015. This leaves me with two options.

The Sony Xperia XZ1 Compact:

I'm still using the XZ1C, it's still fast for what I use it for, and does everything I need. The modem bug annoys me though, and keeps it from greatness. The battery is still really good, even after replacing it with a Chinese battery and 80% charge limit. It's almost the perfect phone. Make it 18:9 and have modern specs with stable software and it would be awesome.

Runner up (The only other post 2015 phone I've used enough and is good enough to like):

I really liked the S7 (Exynos), the last semi good Samsung (until you get to S10e) Although it's kinda slow and didn't work well for cell in the US.

Droid Turbo. I miss that thing so much.

I can't remember if it's pre or post 2015, but my HTC One Mate will always be near and dear. I honestly would've kept using it for other things if it didn't just stop turning on one day lol.

LG v10, honor 8, and my most recent phone, the OnePlus open.

With a 2015 cutoff I think my Samsung S10+ is my favorite. Its only second to the Nexus 4 which is a few years before 2015. Probably going to upgrade in the new year but going to miss that headphone jack to whatever I replace it with.