I'd have to say Rene Auberjonois should win awards for acting through all that Odo makeup.
"The Alternate" The way he's acting just before he starts changing and mutating before Doctor Mora's eyes. It's all in his voice when he yells "NO!" and slaps his hands down on the panel.
"Crossfire" Dude, Odo tears up his quarters when he finds out Kira is seeing Shakaar. I think that was one of his biggest ever RAEG fits that weren't influenced by outside sources. (ie in The Alternate he'd been affected by something gaseous on a planet.)
"The Begotten" EVERYTHING in this episode is beautiful.
"Heart of Stone" Did you see how he almost goes into the fetal position at one point because he's cracking inside at the thought of Kira dying? (That female Founder is a giant bitch for what she did!)