It's like a new guy asking which razor to get.

On the high end are apple and samsung, a little below them are companies like lg and motorola and then you have dozens if not hundreds of others. Apple makes 2-3 models and updates them roughly every year. The others do the same for their flagship model in 2-3 variants, but have many more models at lower price points.

Start by deciding on your budget, then go to your provider's website and see what current phones fit in that budget. You can go to their store to see them in person too.

For the flagship phones it doesn't matter which provider - they make them for every one (which in usa are really four: verizion/sprint/at&t/t-mobile), and these days you can even use the same phone on different providers by simply swapping the sim card (not exactly the same as using the correct phone as some frequencies can be missing, but would function). The other providers like tracfone, h2o, etc. are using the infrastructure of the big one and just repackage the services in a different way.

You probably have realized it already, but one new thing with a smart phone would be a larger monthly bill because you'll have to pay not only for phone calls and text messages, but also for the internet on your phone (you select the data limit on your plan when you sign up). You don't have an option to have just phone calls and not pay for internet/data if you are going to use a smart phone - for that you need to stay with a dumb phone.

You can also complicate things further if you decide to switch providers (you can keep your phone number) but that depends on the coverage they have where you live - the only way to know if it's good is to ask people with different providers about their experience in the places you spend your time. If you're happy with the coverage you get with your current provider the easiest is to just stick with it for now.

Lastly I think most if not all of them ditched the 1- and 2-year contracts, or at least have the option of doing month-to-month service. I think all offer installment payments on the phone itself over 12-30 months, if you're credit worthy, and if you leave them early you have to pay the remaining balance, but no penalty for early contract termination.