A GREAT pin is a combination of theme.. artwork coupled with game play... audio is neither her nor there for me seeing as 90% of my machines have chimes.

I think the real killer of a good design is such a fine balance that many pins fall short... the ultimate goal should never be unachievable, but at the sametime it should be achievable every now and again. It should be a game of skill shot coupled with a hint of luck.
FIREPOWER rates highly as its a game where the goal is simply to achieve multiball... how many FP owners will attest that you can lock 2 balls... but that 3rd ball is a bugger. Thats why FP is such a good game... you are almost there, but at the sametime, off by a mile.
Gameplay and goal should be like a tempting carrot in front of you that you can smell.. touch with the tongue but not quite grab with the teeth.
That is GREAT pinball design and the greats are few and far between