Man it's hard to pick one. For a long time Ice Slick was my favorite, just for the way it turned the battle around. But I also adored Knockout Blow; although some folks didn't like the long animation, I appreciated the extra time spent anticipating the big hit (plus, it gave me time to say "Here it comes, Khan," out loud).
I came late to Stone Melee, but oh boy Seismic Smash was great (and Fault, paired with Obsidian Shield, was perma-stun, never mind the knockdown.)
I'm also a fan of Rain of Arrows and Blazing Arrow. And sometimes Sniper Blast, with Boost Range, was hysterical from high altitude.
For one that hasn't been mentioned yet, Ripper from Spines. I loved the flip and the BOOM knockdown.
For appearances, I loved Storm Kick on a female character -- the epitome of easy grace, yet powerful. And Dual Blades' Typhoon's Edge was the other most beautiful power.
I have to say I liked Lightning Rod a lot more when paired with Shield Charge and Thunder Strike. BOOM, BOOM, BOOM is better than just BOOM.
Perhaps my all-time favorite was Lightning Storm
at the top of a stairway. It would repeatedly throw enemies back down the stairs onto their backsides every time they tried to climb up toward me, and it wasn't too hard to keep two of them up continuously.