I think some of the best execution has to be on the Irish passage graves, but the most amazing (variety-wise) is certainly in GB.
I am constantly amazed at what I see appearing on TMA in the rock-art dept.
I think the vast majority of Irish rock-art panels are buried under 6ft or more of peat and will never been seen again. I tried to find some on Sunday that was visible 15 years ago, but is now covered by sod.