I've got two D15s and a D15S for point of reference. I'll compare it here with alternatives like the Silver Arrow IB-E Extreme, NH-U14S, D15S, Cryorig R1 Ultimate and the Dark Rock Pro 3.The D15 deserves its reputation. It is, without any doubt, the most powerful cooler for the real world. It's true that the Reeven Okeanos can keep up with it, and the Silver Arrow IB-E Extreme can even surpass it, but in the real world, owners won't put up with coolers that loud. Those two are leveraging high RPM fans to compensate for inferior efficiency, and as a result, most owners are going to clamp down on fan speed and end up with lower performance coolers. The D15 is so quite, even under full load, that inside a case it might as well be silent. Its full performance range is, thus, accessible in the real world. If you're alright with loud heat sinks, I have to imagine that using two Noctua NF-A14 iPPC 3000s would still defeat all comers. I'm not sure how well those fans would fit given the NF-A15's (stock fan) strange frame.In my view, the Okeanos and Silver Arrow don't offer serious competition for this because of the way they achieve their performance. Far more interesting, for a comparison, are its direct competition from Be Quiet! and Cryorig, namely the Dark Rock Pro 3 and the R1 Ultimate. The R1 Universal, in my opinion, is the direct competitor to the D15S, so I won't get into it much here.The Dark Rock Pro 3, while it is measurably quieter, is simply not in the same league of performance as the D15. On top of this, it's harder to install, and in practical terms, both are virtually silent anyway. The DRP3 has more in common with the outgoing D14 anyway, with its dissimilar front and middle fans for memory clearance.The R1 Ultimate poses a much bigger challenge for the D15. It is ever so slightly behind the D15, and a little louder, but in some ways it's more, and in some ways it's less, compatible. The R1 is significantly taller than the D15, so people with certain case limitations will be left out. However, it's also narrower than the D15, meaning not only is it more friendly to the neighboring PCI-E slot, it's also easier to reach down and unlock the GPU from the motherboard. Furthermore, it overhangs the memory less, although due to the way the fans are mounted, it may still have less clearance than the D15.Perhaps the R1 Ultimate's biggest weakness, relative to the D15, is the fans. The XF140s do look great and are actually quite good fans, but they're still using modified sleeve bearings that will negatively impact noise and longevity, as well as severely limit orientation (not a problem for most builds).Although the D15 is probably the most powerful air cooler ever made, at least in terms of noise levels that normal people would find tolerable, it does have several weaknesses. The first is its extreme width, which I've already discussed. On some of my more poorly spaced motherboards, like my new Crosshair VI Hero, I actually have to remove the middle fan to reach the GPU lock.The other is, I'm sad to say, quality control. I've had many Noctuas before and never found an appreciable scratch. But I've had two D15s and a D15S and all three have had minor blemishes. The first D15 had a small, but deep, scratch on the top plate/fin. The second had other, smaller scratches, although these are not really noticeable. The D15S was the worst, which had a small, but easily recognizable, corrosion spot on it, and it's being returned now. I can only guess that the increased surface area, probably about twice that of the next largest current-generation Noctua cooler, the U14S, makes it that much more likely for a scratch to occur. Still, I've come to expect better from Noctua.That said, you simply cannot find an air cooler that puts out this performance at this volume. There is almost no reason to use anything else in general. That's why it gets 5 stars. Although it's arguably the most powerful, is it the best?Probably not. I'd say that, depending on your needs, the D15S or the U14S is better. The D15S comes with only a single fan, but it moves the heat sink away from the GPU, making it easier to work on the computer and clearing the GPU on smaller motherboards. The U14S has infinite RAM compatibility on both sides, making it easier to work on your computer and allowing you to use any RAM you like. Thus, depending on which compatibility issue concerns you more, I'd go with either a D15S or a U14S. Each gives up between 1 and 2 degrees Celsius in overclocked loads, due almost exclusively to being single fan heat sinks. If you want D15 performance, simply add a second fan to the D15S, and a dual fan U14S will get very, very close to the D15. In many tests, I've ever seen dual fan D15Ses beat D15s, although it's unclear why that would be. Perhaps they're better aligned with the intake and exhaust fans in the case. Perhaps they are less affected by the heat coming off of the GPU. I'm not really sure, but suffice it to say, you can achieve D15 performance without some of the hassles of the D15.I've added photos of the D15 (non-S) with and without the front fan to give you an idea of memory overhang. For compatibility reasons, some people may decide just to go without the front fan altogether, with also works fine.