I accept Buffy's account of Heaven. I just believed Buffy would rather be fighting in the world than slumber in the afterlife. Being proven wrong hurt, and it somewhat diminished Buffy in my eyes.
You know, I've never fully got around to this as well. Like you, I can
fully empathise with Buffy's pain. I also fully understand that her experience of the real world was hellish, and something she wanted to escape from.
However: the fact that she was
OK with the idea of being permanently dead, even
preferred being dead in 'heaven' forever, felt not very Slayer-like to me. Don't get me wrong, it's a very powerful metaphor and it's very Hamlet-ian... personally I think it was a good authentic touch and made the Slayer's experience of trauma feel very real (the loss of her mother, the endless battle a Slayer faces, the responsibility for others, could lead to her feeling suicidal from the pressure and sadness). But being dead isn't what the Slayer ever wants, or ever wanted in the long term, so Buffy's feelings while very real, were wrong at this point. This is confirmed when she admonishes Cassie Newton in S7's 'Help' for her laid back acceptance of her impending death "that doesn't sound like someone who wants to live!"
I always read that to mean that while Buffy could be justifiably exhausted from all the pressure to "happy" at being brought back, I still think in the long term Willow's actions in bringing her back from the dead made her grateful. Not at the time when it happened, but in the future.
And so as not to derail the thread: I love
all the Buffies. Don't even have a least favourite.