Armada Unicron. Close second goes to ROTF Devastator, who mostly loses due to Unicron's horns, but beats Unicron in sheer bulk.
Brave Max, 20th Anniv Prime, Cyb Primus, Supreme Starscream from Cyb. Oh, and my 20' tall self made Prime I keep in my backyard. Just kidding about that last one, but that would be cool, if you could ignore the weather.
As of this moment, Masterpiece Megatron MP-05 towers over the rest of my Transformers. His Barrel alone stops about 3 inches above Masterpiece Optimus Prime MP-01
Armada Unicron. Or DotM Ultimate Optimus Prime's wings, one or the other :/ Actually, I haven't made a size comparison yet between RotF Jetfire + Optimus Prime and Armada Unicron, though if the former is taller it's only because of wing/thruster height from the Jetfire part. And if the it's taller then that's my tallest TF.
Im not sure if PCC bruticus is taller than Rodimus with Protector armor, but if so, he is. Until I get G2 Bruticus in November. But Im getting Primus in like three days, so easily Primus