Never saw Yamazaki 20 but I've tried the 10, 12, 18, 1984, undated and Puncheon. Out of all of them I like the 12 best, which is probably good because it's easily available over here for about $58 a bottle, unlike the others.
The undated stuff and Puncheon both tasted like once was enough, as did the 10 year.
Pricing in Japan:
Yamazaki 10 ￥3,780
Yamazaki 12 ￥6,480
Yamazaki 18 ￥17,800
Yamazaki 25 ￥89,800
There was no pricing available on the 1984 as it is no longer available, but for about $20 I got a glass of it! Still prefer the 12, but that's personal tastes, and I let my friend finish it because he seemed to like it better than I did.