Here are some of the best unlocked smartphones to use with a prepaid cell plan or replace a phone before a contract is up. Prepaid cell plans are a great value, but they require you bring a compatible device or purchase a new phone at full price, and the carrier may not have the greatest selection. Some manufacturers make it easier than others to purchase unlocked versions of their phones online. There are also a few great unlocked smartphones that are sold at very reasonable prices compared to a new iPhone or Samsung Galaxy phone. I'll go over some things to look out for, the best models, and where to buy.
GSM vs CDMA
For technical reasons, some carriers require that you purchase a phone directly from them (though you can probably find a compatible phone on eBay if you have to). This applies to plans that run on Sprint's network such as Virgin Mobile, Boost, and Republic Wireless and plans that run on Verizon's network.
Newer unlocked GSM phones will most likely be compatible with both AT&T and T-Mobile, and the prepaid carriers that run on those networks. That includes Cricket Wireless and Straight Talk, the two best smartphone plans in my opinion. The rest of this article will focus on GSM phones and plans, due to their interchangeability and better selection of prepaid plans.
Best unlocked smartphones at the high-end
I'll start at the high end of the pricing spectrum, and work my way down. Apple makes it easy to buy an unlocked iPhone online. Select the 'unlocked and contract free' model. I recommend getting it with the T-Mobile SIM card regardless of the carrier you pick because it will save you $10 if you ever decide to move to T-Mobile. The iPhone 6 starts at $650 unlocked, but the 5s is available for $550, and the 5C for $450. Unlocked iPhones are also available on Amazon, but the newer models tend to be priced higher than the Apple Store.
Google's Nexus program has traditionally been a great place to get a very affordable unlocked phone, but the Nexus 6 starts at $650. With a 6" screen, it is more comparable to the $750 iPhone 6 Plus than the iPhone 6, though. It is also available on Amazon, and is actually $40 less as I write this.
The Motorola Moto X is one of the highest rated premium Android smartphones, and is priced a little lower than most. It is available for $500 on Amazon right now.
Great mid-range unlocked phones
There are several mid-range unlocked smartphones available at very reasonable prices. OnePlus makes a great phone starting at just $300. You'll need an invitation to order one, and they aren't always easy to acquire, even months after release.
Motorola makes a few very affordable unlocked smartphones. The Moto G starts at just $180, but doesn't come with LTE. The last-gen Moto G with LTE starts at $200. The Moto E is a very capable smartphone priced at only $150. If these Motorola prices seem too good to be true, read some reviews to see just how capable of a device can be had for under $200 these days.
There are plenty of other options for great unlocked smartphones on Amazon, from last-gen brand name models to manufacturers you may not have heard of. Just make sure the phone is compatible with your carrier before purchasing.
Nexus and iPhone, two of the best unlocked smartphones
Prepaid and unlocked - save money and gain flexibility
Even after spending $650 or more on a premium unlocked smartphone, a prepaid cell plan can save you hundreds over a traditional plan with a two-year contract. Pick one of the many great mid-range unlocked smartphones and you'll save even more. I have been using a prepaid plan for over two years with an unlocked iPhone 5, Nexus 5 and iPhone 6. I've tried several different prepaid plans, which is one of the great things about having an unlocked phone. I enjoy the freedom of being able to change plans or devices at any time and highly recommend it.