Another vote for 7-11.
All cash. What I like about it is that the time only expires after 1 year. You just buy time as you need it. It's not a monthly bill.
But that said, a burner offers you some anonymity, but don't think that if you do something that attracts the attention of LE that they won't find you. I'm sure that the phone will have some sort of serial number which will be known to the telecom compnay every time you use the phone and it it's an "old phone" that used to be your day to day driver, well, that phone and all of it's numbers all tie you to the phone.. When you use your old phone, the teleco will track the phone number, the sim card number, the phone ID number and probably 5 other numbers all at the same time with every call. And if so if you're using a phone that was somehow linked to you, well, so much for anonymity. In addition, if they have the numbers, they can locate the phone (easily) and therefore, locate you. Probably don't even need a battery in it and they will be able to locate the phone. Like a glorified air tag and they will even know where you've been and where you've used in the past.
At best, a burner phone is great for when you want to send or receive calls where you aren't doing anything illegal (shall we say), but you just don't want whoever to be able to have your day to day phone number.