It's pretty basic by most people's standards (I think it was designed to be a bridge between normal phones and smart phones) but my trusty lil' Nokia 6730 Classic is what I've been carrying the last couple of years. It's nice and slim, has decent battery, looks and acts like an actual phone, doesn't have any touchscreen or tiny keyboard nonsense and focuses on being a phone first and everything else second.
It's running Symbian S60v3(.2), has about 120mb of RAM, a 600MHz processor and a screen resolution of 240x320 off the top of my head. It doesn't look or behave like a smart phone but heck, I can get Doom running on this thing along with a few other old games, I've got a SSH client and even a pretty old version of python ported to it in case I need to program on the go or ever get an itch to code something for it. Mostly just use it as a phone (i.e. calls and texts) though
It's starting to act up as of late but hey, I got this thing free as part of my contract and it's more than enough for my purposes on the whole.