It depends.

 If you want the highest performance and efficiency, the Bitmain Antminer S9 models - but they are widely known to have reliability issues.
 If you are more worried about reliability, the Caanan Avalon 741 has a very good track record, at the price of a little lower efficiency and lower performance than the S9.

 EBang BW.Com and Hotmine don't have as many units in the field in the hands of non-large-mines so their track record is a lot more "unknown" except for the large farms that have lots of their units.

 Innosilicon dropped out of making SHA256 miners entirely, but the ones they DID make were rock-solid reliable - it's a major bummer that they didn't move into the 14/16nm generation.

The idea is that Bitcoin is "old tech". by adding smart contracts it now competes with "new tech" Ethereum.

Don't kid yourself. Bitcoin is pseudo-anonymous.
Anybody can see which unspent outputs are used as inputs by a new transaction, and which address got funded. By seeing which unspent outputs are combined in a transaction, addresses can be coupled to wallets.

Since a lot of exchanges started to use KYC regulations, and everybody and their mother are logging ip's, allmost all your exchange activity's can be traced back to you.

To make things worse, as soon as you buy a physical good using BTC, you have to enter a valid address, pointing directly at yourself.

Even mixers don't provide 100% safety, since you don't know who's operating them, and by the ever increasing field of data mining and blockchain analysis, to some point, it's even possible to join together mixer's inputs and outputs (or so it seems... I've never done this myself, but i've read that theoretically, it's possible).

As for the OP's question: i've bought some marihuana seeds and a knife using BTC... It's not really illegal in my country (they were both purchased on legal webshops, using my real name and address), but those are the most grey-area things i've ever bought with BTC