Pulstar is infuriating at times, but to me Magician Lord, Magical Drop III* and yes, Nam 1975 round out the top 3. Magician Lord needs no further explanation. Nam 1975 is crazy at home, but even in the arcades that final boss is still a huge pain to beat.
Magical Drop III I say because I always play only one credit, and I always get hundreds of thousands of points, meaning I have to face Black Pierrot as the 12th enemy, and at that point he is impossible. It truly belongs in the "puzzler impossible to beat in a quarter" category, along with Pop 'n Bounce.