Someone else answered it exactly how I would have answered it so I didn't bother, but I'll repeat their answer.

Silkroad could not exist without Bitcoin. There are sites like Silkroad that do not use Bitcoin, but they aren't Silk Road because they don't use Bitcoin. Only Bitcoin offers the features required for a Silk Road.

So the only remaining answer to your question is Bitcoin - but a Bitcoin which made Silk Road(s) impossible would not be Bitcoin. It would be some other currency that made arbitrary moral decisions about the acceptability of transactions. At that point we might as well be using Flooz.

In other words, your question is completely unanswerable because it's a false dichotomy.

Now answer my question. What would be the purpose of a "regulated Bitcoin"? That is, what would be the functional difference between (I'll use the US here) the USD, which is a regulated, centralized, electronically transferrable currency issued by a central authority, and US-Bitcoin, the centralized, regulated, electronically transferrable currency issued by a central authority with a blockchain?