At the risk of challenging GCA's resident tastemaker, I find 2, 4, 9, 12, and 15 to all be outstanding holes, and I'm tempted to include 14 in that group as well.
One of the hard things about evaluating Erin Hills is that its combination of excessively minimalist design and extreme elasticity makes it hard to evaluate holes individually. 4 is a perfect example - it's a hole that can play anywhere from ~350 to ~450 yards, with the challenge primarily provided by a combination of gently rumbling land, an intimidatingly shallow green, and a smattering of bunkers that are all in play in different ways on a given day depending on the day's tee and hole location. It can be a very different hole from one day to the next, and I suspect we'll see that in the US Open if the presentation of it during the US Amateur was any indication - Mike Davis will surely screw with tee and hole placements to vary the way the hole presents itself over the course of the tournament.
Contrast it with a hole like the 13th at Augusta, which is a tricky hole with plenty of scoring variation but with a much clearer playing challenge: Hug the hazard off the tee to cut off as much distance as possible, and then either take on the risk of hitting a well-defended green or play safely short and try to get up-and-down for birdie. The challenge isn't easy, but it is relatively straightforward to explain to someone like my wife who doesn't understand golf at all.
If we all play the 13th at Augusta in our imagination, we all probably play it about the same way - a perfect draw around the corner off the tee to set up a heroic second that we land safely on the green to set up our eagle. If we play the 4th at Erin Hills in our imagination, the differences between what we envision begin at the tee (which one are you playing!?) and the route to the cup each of us takes will largely depend on which side of that green we'll eventually pull our freshly-dropped birdie putt from. And the 4th is far from the only hole at Erin Hills that I could say this for - for proof of how hard it is to envision the "correct" play on some of the holes at Erin Hills, one doesn't need to look any further than Patrick Cantlay's meltdown on the ~350 yard 15 in the 2011 US Amateur when he overthought things after the tee was moved up a bit, pulled out an 8 iron on the tee, dumped it into a centerline bunker, and began the unraveling that would cost him the tournament. If I close my eyes and picture playing the 15th again, I spend a solid 20 seconds just planning where to hit my tee shot. I don't even know the play when I'm just daydreaming! When I close my eyes and picture playing a hole like 13 at Augusta, the daydream doesn't even begin until I'm already at the top of my backswing on the tee shot...
I don't know if that means 4 or 15 at Erin Hills are great holes or not. But I do think it makes them interesting holes, and I know I really enjoyed playing the course. I didn't necessarily want to run back to the first tee again - it's not a 36 or 54 hole kind of course - but as someone who loves to replay shots in my head with the endorphins from a tired pair of legs firing when I head to bed after a day of golf, I was as satisfied falling asleep after playing Erin Hills as I have been after any course. Sure, it's too long and has a few too many elevated tees and greens and the walk is often exaggerated but still a tough one. But it's also unlike (almost) any other course I've played in terms of its ability to present a variable challenge. It's a shame that it's an expensive resort course and such a challenging one to score on and walk at that. It stores a lot of secrets that would be a joy to unlock through multiple plays.