Third Romatiklan, by a wide margin, assuming Keevan doesn't count. Rocks and Shoals is one of my favorite episodes. I also like Mareel in Invasive Procedures. She beat up Kira - who attacked her - without breaking a sweat, and rubbed it in a couple scenes later by asking her "are you ok?" in a mock-caring, motherly tone upon seeing Kira try to walk across the room with obviously sore ribs.
No Altovar was the guy who tried to buy the Gel and psychically attacked Bashir forcing him into a deadly nightmare where Altovar haunted him. The real Altovar was captured almost immediately after the attack in the episode Distant Voices. Grathan Tolar was a criminal on Kingon death row who Sisko spared to make a Holo Recording on a Cardassian Optilithic Data Rod. The rod was obtained by Garak who paid for the rod using Bio-Memetic Gel given to him by Starfleet over Bashir's objection in the episode In the Pale Moonlight.
Jaro (if count story arc as one; mainly because such a watchable actor). But also love all the Vorta (Vortaes?), and Omet'iklan, who seemed as badass as Sisko.
I would definitely agree with Jaro. Good call! I agree that it should be counted as one episode. It's one continuous story, and he hasn't popped up in random episodes since. It counts, in my book (for what it's worth)