Panzertank PM4

This synth is ancient, but I haven't found a better one for learning FM (phase modulation and frequency modulation are identical with sinusoidal operators - the DX7 was phase modulation, marketed as FM because FM is conceptually easier to wrap your head around).

How to pickup FM is one of the most common questions among electronic production communities. Most people make it much too hard for themselves by watching confusticating tutorials and starting with fully featured synths like Dexed. All the tutorials, user manuals, wikipedia pages in the world don't hold a candle to simply locking yourself in a room with a simple synth like our guy Panzer here.

There are only 4 operators, each operator's parameters are vertically lined up (making it easier to visualize what's going on), and schematics for each algorithm (the different routing configurations between operators) are right on the panel for quick reference. You have no excuse for not having learned FM, and this is unbeatable for learning FM on.