Are we talking appearance or acting skill? The truth is that I don't know if I've ever seen a perfect 10 performance.
In movies, I loved Philip Seymour Hoffman's portrayal of the title character in Capote. I thought that was one heck of a well-deserved Oscar. Vivien Leigh and Clark Gable really had me in Gone with the Wind, too. Vincent Price in Witchfinder General. Also, Emily Watson totally rocked Breaking the Waves. Gulietta Masina (sp?) was wonderful in La Strada. Adrien Brody delivered a masterful performance in The Pianist. Oh -- and Kenneth Branaugh is a genius when it comes to Shakespeare.
In TV, I've got to say that James Marsters as Spike is way up there for me. Tommy Lee Jones and Robert Duvall as Captains Call and MacCrae in Lonesome Dove, everyone on the cast of My So-Called Life, Halle Berry on Queen, and Sarah Brown, Rick Hearst, and Jonathan Jackson on General Hospital (consistently, with John J. York, Sebastian Roche, Nancy Lee Grahn, and Maurice Bernard putting in the occasional mind-blowing performance) are also near the top of my list. I really love the physicality of these actors' portrayals of their characters.
I can't put my finger on the best acting I've ever seen but, if pressed, I think I'd have to say that my current and most consistent favorite is Rick Hearst, who takes even the crappy lines they give him all the time and knocks them out of the park 99% of the time. I am totally in love with his acting.