An acid bath and a brush.

But I can't help but wonder if your question is something other than what you said. For the engine *block* is a hunk of iron that isn't subjected to much heat. It can be rusty/crusty/nasty and work just fine. A block can be hot tanked. A block can have a brush run down its passages and be visually examined.

But a complete vehicle, with an aluminimum head and a copper radiator has different requirements and different issues. Dumping strong acids into a vehicle cooling system that would clean the iron block will happily dissolve the radiator and hoses. Something you probably wouldn't want.

Generic off the shelf flushes all work about the same, using about the same chemicals. If you're really into yellow bottles, buy Prestone. If you like black, buy Champion. If things are really nasty, none of them are going to work well.

Simply removing the hoses and thermostat and using a garden hose backwards and with enthusiasm does a very good job of removing sediment and lose filth that's laying in the bottom of things. It's not going to remove scale.

Never tried this, but I've long looked at products like CLR and wondered what they would do to scale inside the radiator and the like. Might try it some day on a vehicle I don't care about. But be aware of the potential to disolve things more than expected.

So what problem are you trying to solve?