Generally what people already said in the thread is the most useful advice - pick heroes that have multiple forms of CC that interrupt his Dark Conversion. The idea is that you can use one of your CC’s to still engage and do your job, and save the secondary one specifically for Dark Conversion. Also you’ll have a good tell of when he wants to use it since he will always want to when he’s at low Health, so you usually have a generous window to look for it if you constantly keep it in mind.
Some heroes that are great to interrupt it:
ETC - his Q and W are fast and easily able to knock him out of his channel
Johanna - Her W and Blessed Shield can be used for an interrupt. Her trait can also be used to try to survive if he gets the ult off on her. This is a little more risky/takes more skill as her W is the main form of an interrupt so you have to pre-emptively cast it as he’s getting low.
Diablo - He has his Q and E to interrupt it quickly. He also has various talents that can reduce the cooldowns of these abilities to ensure that you have one of them available when you need it.