To the uninformed, Bitcoin is synonymous with hackers, fraud and money laundering. This is largely due to a lot of cryptocurrency's exposure coming from stories of large scale darknet marketplace takedowns and the ongoing ransomware crisis. The truth of the matter is that Bitcoin is not the ideal currency for the underground.
Cybercriminals do not Stick to One Thing
Cybercriminals have and will always use whatever is the most convenient means to suit their needs. Back in the early 2000s—long before Bitcoin and our modern cryptocurrencies—carders and fraudsters favoured a (now defunct) digital currency called "e-gold". E-gold was very convenient at the time; making it viable to run international Ponzi schemes, launder money digitally and shift the hacker underground from the "just for lols" pranks to the modern cyber-criminals we know today. Many forums for carding were operated: CardersMarket, DarkMarket and ShadowCrew to name a few. However, law enforcement caught up with underground and cranked down on e-gold by 2006—having already taken down many of the available carding sites already. For the underground, it was time to migrate. Enter Bitcoin.
You can read more about E-Gold here — https://www.wired.com/2009/06/e-gold/
Why was Bitcoin Interesting to Criminals?
Bitcoin was designed to provide a decentralized and pseudo-anonymous currency exchange for everyone. Much of Bitcoin's early popularity was in no small part due to the rise of Ross Ulbricht's Silk Road—the infamous darknet market—and its subsequent takedown. Users of the marketplace had to learn about and purchase Bitcoin in order to buy the drugs and services they wanted. Silk Road—being the largest darknet marketplace at the time—inspired others to incorporate Bitcoin into their hidden services. Thus, leading to a wide adoption of Bitcoin in the underground community.
The media catching onto Silk Road and reporting on its takedown brought Bitcoin into the mainstream with promises of protecting your identity while purchasing online. Regrettably, the early mainstream attention of Bitcoin came with the stigma of being associated with the mysterious "Deep Web" and criminals. Our beloved coin was used extensively by criminals in their day-to-day operations. Suprisingly, some articles imply that criminals are dropping Bitcoin—such as the article linked to below. Why is that?
"Criminal underworld is dropping bitcoin for another currency" — https://www.smh.com.au/business/markets/criminal-underworld-is-dropping-bitcoin-for-another-currency-20180102-h0co06.html
In recent years, Bitcoin exchanges and vendors have put in place regulations to prevent credit card fraud. When buying coins with a credit and debit cards, users are required to submit documents verifying their identity and provide proof that the card they are using does in fact belong to them. This has curbed money laundering attempts and card dumping with mainstream vendors such as Coinbase.
There have also been efforts to de-anonymize Bitcoin users in order to help put offenders in handcuffs—much like the federal crackdown on TOR users to prevent acts such as the exchange of drugs and child pornography. In wake of recent safeguards, the criminal underground will not grind to a halt over the regulation of one coin as history has shown. The reports of mass migrations to Monero are over-generalizations concocted to sell newspapers and drive subscriptions.
So, what is the official currency of the underground as of 2019? Short answer, there is none.
As said before, criminals use whatever is the most convenient for their day-to-day operations and there are so many to choose from nowadays. Different types of criminals need different types of digital currency since there are so many in circulation to choose from. The most popular of these amongst criminals are the most popular mainstream cryptocurrencies: Ethereum, Litecoin, Monero and Bitcoin still. Once the authorities catch on to the current techniques used by cyber-criminals, something else will be used and this circle of life will continue.
In short, do not believe news media when it comes to cyber-criminals and cryptocurrencies. Don't drop your coins for the newest, trendiest altcoin. Statistics don't lie, but it's possible to lie with statistics. Any currency has and always will be used by criminals because they are human just like us. Be very wary and understand that reporting the bad sells newspapers while the many legitimate uses get buried for newcomers.