640, 650, and 550. 550 will probably be my next phone. 640 is good, but it is running WP 8.1. The thing about WP 8.1 are the idiosyncrasies between different iterations of the OS. Like the difference between Cyan and Denim. And the fact that a lot of us cannot get Denim because our carriers block it. So I lean towards the 650 and the 550 for that reason alone. Sure, you could get an 8.1 phone under $100 but why would you, unless you just need a phone, like, yesterday. I already purchased a 635 for $40. I'm riding this one out until it doesn't turn on anymore.
If you can get a 640 for like $20, which is what it was through GoPhone during Christmas, then by all means. But if you don't need the phone, or can afford to spend more, I would. Granted, Windows 10 is still buggy, and going through some issues, but the apps, particularly the system apps, are far superior to those on WP 8.1. You'll recognize most of them because they should work as well as whatever Microsoft apps you have installed on your Android device.
Conventional wisdom says get a 640 for less than $50. Understandable. But 8.1 is the past, and 10 is the future. Its like suggesting that we try out KitKat or Lollipop. You really won't know where Windows Phone is going if you're using an 8.1 device. Sure, you could update it through the Technical Preview but why put yourself through that.