It depends on the carrier. (ST is a virtual network operator, not a carrier.) If you want AT&T or TMobile, the best LEGITIMATE deal you can find. (Chinese "clones" that are "clones" of phones with much less capability - and zero tech support - aren't bargains. Neither are European phones that don't have he radios needed for high speed data in the US.)
If it's Verizon, any Verizon phone that will pass the test at
Straight Talk Compatibility Tracfone: BYOP For Sprint, Walmart or Straight Talk. I don't think they do BYOP with Sprint yet.
BTW, don't compare he "full prices" you see for the phones with the prices the carriers charge for the same phone. If you're paying the carrier $50/month more for service on a 2 year contract, you're paying $1,200 more for the phone. No legitimate, major manufacturer's top of the line phone sells for $400. You pay for the phone by buying it (most of the world does it that way) or you pay the subsidized price and pay for the phone by the month because your bill is so much higher. That's why ST charges you $45 for what the carriers charge you $95 or more for.
In cellphones, your first choice should always be the carrier - which one has the best coverage where you need coverage. A great phone with a carrier that doesn't have service where your house is located isn't worth much to you.