It is hard to pick the easiest or hardest holes when the can play so different from tees, with wind, etc.
9 at St Andrews is probably the easiest hole on WGT from any tee and with any wind. I would have to agree with that. You can also make a case for 10 and 12 at St Andrews as well, those holes are reachable from any tees except the legend tees and even from those from time to time.
7 at Kiawah is a very easy hole from tee to green but putting can be difficult around the hole you might put 11 at Kiawah in it's place since the green is much more simple.
1 at BPB is a very easy hole if you put your drive in play.as is the 8th hole there (I think) the one with the sloping fairway, a good tee shot leaves you a simple 90 yard shot to an easy green.
Everyone has their opinion about this, not sure that I would even want a best of easiest WGT holes. The most famous is a good concept as is the hardest but that is so tier specific. You have holes like 13 at Kiawah that are 100 longer from the Legend tees than master, I think that is one of the hardest holes but from the master and up tees, it's only a drive and a short wedge.