I don't think I've ever seen another actor become a character as much as Heath became Ennis. I mean...yeah...there are plenty of actors out there who have a defining role, a character we'll always think of when we hear their name, but I still see the actor on the screen in most of those cases. That's not what I see when I watch Brokeback - it's Ennis up there, 100%. The mumbling, the mannerisms, the haunted look in his eyes - none of that is Heath Ledger (well, maybe the mumbling is a little bit but he normally mumbles in an Australian accent

) There are plenty of times that Ennis doesn't even look like Heath

As a Gyllenhaalic, I should probably say that Jake isn't far behind either, although his looks do make it harder to escape who it is up there...especially when you take into account the description of Jack Twist from Annie's story. But he still does a masterful job of conveying the hope and, ultimately, the despair that Jack endured for his love of Ennis and I can't think of any other actor who could have done the job better than Jake did. It may not ultimately be his defining role (some may even argue that his performance in Donnie Darko was, or that his portrayal of Robert Graysmith in Zodiac is) but for us at least, Jake and Jack are forever one and the same.
The only other performance I can think of that comes anywhere near Heath's, though, was Liam Neeson's portrayal of Oskar Schindler in Schindler's List. If he had been disguised a little then it would have been the perfect performance but, while his acting convinced me completely, in looks it was still unmistakably Liam Neeson. Then again, he's not the easiest actor to disguise!