The phrase "Most thorny" is correct and usable in written English.
It can be used to describe a situation, issue, or problem that is particularly difficult or complicated.
Example: "The negotiations became most thorny when discussing the budget allocations."
Alternatives: "Most challenging" or "Most difficult."
The Paris treaty left unresolved the most thorny and difficult territorial question, that of Trieste.
This is not surprising: it is the most thorny terrain in politics.
Another major polluter was Ciba Specialty Chemicals Corp. in Toms River, once Ocean County's biggest employer and now one of New Jersey's most thorny and costly Superfund sites.
The stalemate continued to be caused by one of the most thorny of Iraq's problems: the divided city of Kirkuk in the north.
The highly volatile issues to be resolved include the demarcation of the border separating the nations, the status of citizens in each country and, most thorny of all, oil.
The second most-clicked story of the year tackled one of the most thorny of genetics questions: whether and how variations in intelligence can be traced back to variations in genetics.
The range is huge, from the simplest, folksiest, most unpretentious to thorny, abstruse and high-minded.
Indeed, even as Mr. Ryan calls for bipartisan agreement on the most important — and thorny — domestic political issues, his own proposals strike at the heart of Democratic orthodoxy.
Mr Blair can be a little thespian for some tastes, but Mr Johnson always manages to sound like a normal human being, even when dealing with the most complex and thorny issues.
THE assassination of three Kurdish activists in Paris only days after the Turkish government announced it had started peace talks with the jailed leader of the Kurdistan Workers Partyy, the P.K.K., underscores the difficulties of finding a lasting solution to Turkey's most acute and thorny internal problem.
From a microphone, the commissioner delivered to the half-filled room what had become the most important, and thorniest, public message of his career.
