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Any fungible good can be used this way, like regular cash, which is what's used for the vast majority of crime.

But there is a certain amount of law enforcement involvement with cash too. Fighting forged bills, tracing bills by serial number, etc.

Tracing serial numbers on Bitcoin is way easier

https://darknetdiaries.com/episode/ podcast #70 is a very good story on how someone use the prepaid cash on the dark side.

All other stories on that podcast list are very interesting.

Also things that can easily be sold off for close to face value, like iTunes gift-cards.

I think the IRS scammers still usually ask for something like that instead of cryptocurrency, because cryptocurrency is a bit too hard for their marks to figure out.