• Holy Roman Emperor Rudolf II
• Queen Elizabeth I of England
• Paracelsus (birth of pharmacology... yep =alchemy... so TONS of noteworthy physicians were alchemists)
• Popes.. dammit, I'd have to look that one up. Sorry, but yes, there was an alchemists lab in the Vatican!
• Al-Ghazali and Al-Rhazi are knows for much more than just alchemy.. but talked about them on my show.
• Roger Bacon
• Various nobility and Kings. Tycho Brahe, as an example.
• Johannes Keppler! (though more as Brahes sidekick, he was less interested himself)
• "King" (Queen) Christina of Sweden
• Hennig Brandt.. but he's a pretty mainstream alchemist. The line just gets blurry between "famous chemists" and famous alchemists.
• If you're interested in church history, some noteworthy monks and bishops were alchemists... others forbade it (same as popes)
• Since it was taught in Harvard, but already discredited as a practice, you never know who secretly studied it in the American Colonies... but we did a few shows on that too.

Outside of Emperors, Kings, and Popes, though, Isaac Newton is probably the most famous. But your astronomer, physician, or mathematician in the early modern period--is my alchemist. So other way around... pick a famous name in the history of science. Chances are they weighed in on alchemy to some extent. It's broader than you might think. Kepler commented on "optical chemistry" which Newton picked up on.

Many examples of alchemical ideas made it into other scientific disciplines (like distilling alcohol, for instance. So Jim Beam and Jack Daniels? Alchemists--in the sense that they are using alchemical means to purify "spirits" ...if that isn't alchemy, nothing is. We can go far down this rabbit hole though, and probably isn't the intent of your question.)