Haven't seen link round these parts in forever.
Anyway. My vote goes to the guy who did the novaguard plat solo. Archire I think?
Achire, indeed. One of the most impressive solos I've ever watched, for sure. Especially for the time it was done...
On topic, it depends on what you mean by impressive.
If you mean that I was impressed when I watched it, then I guess it's Sp3c7er's or however it's written, and Achire's. But that's also because when I watched Sp3c7er's one I was a complete noob and watched in awe at how he RHAd Banshees to death while the only thing I could do was run around the battlefield screaming in fear of them.
Judging with the experience I have now, and by today's standards... That solo is far from being impressive.
So if you mean which is the solo I've watched that, in my present opinion, is the best showcase of gameplay skill, then I'll go with Alquinn's.