The nzxt kraken x62 generally gets the best reviews. However for the reasons you outlined above, I generally prefer air over water. The Noctua nh-d15s will be within a couple of degrees of the best water AIO systems and beats most of them. There is no pump to fail. It is simpler and more reliable without much of a compromise in performance.
As for your radiator.... I would suspect your pump before I suspected your radiator. You can disassemble your system and do a flow test both directions in your radiator and if there is a blockage that would show it and likely dislodge it. But given that this is a closed system, the odds of that being the cause are pretty remote. While the odds of the pump suddenly dying in these AIO systems is pretty good (common issue).