It's hard to say "which" Luke is the best (IMHO) since he's such a transformative character throughout the OT. When we first meet him in ANH, he's just a whiny farmboy looking to get out of his chores and have an adventure. But by the end of ROTJ, he's willing to give up his life for just the chance to redeem Vader (the consistent villain of the OT btw). In my mind, that's what makes him the movie series' hero - not that he was the one who happened to blow up the Death Star, but that he matured into someone who made the conscious choice of self-sacrifice, and strode into it boldly.
Okay so maybe my answer is lending towards "ROTJ" :P
I guess what i'm trying to say is that I feel like the whole OT is about Luke "becoming" the hero, and it's difficult (in my opinion) to appreciate only one "stage" (or movie) without seeing it through the lens of his entire (on-screen) life.