Looking over my comments in the recent thread on the Mystery site (which I will be revealing properly soon), and especially what I said about how I regard it as the third best videogame site of all time, I decided to make a list happen, and here's what I got so far:
1. Insomnia (see here)
2. Postback (see here)
3. Mystery site (see here)
4. Blame The Control Pad (which is basically HG101 done right, see here)
I also know that Insert Credit will be included in the list at some point (the early IC at any rate, not the rubbish recent revival), perhaps somewhere between no. 5 and 10, once all the sites with proper substance have been included.
On another note, Seth Killian's and Randorama's stuff is also top 10 material, but Seth's stuff was originally posted on a forum (SRK), whilst here we are ranking sites and not individual authors, and SRK is most definitely NOT one of the best game sites ever; while Rando's stuff was originally published in an utterly fucking shitty fanzine called The Gamer's Quarter, from which I rescued it (the early stuff, at any rate, because he has since written episodes exclusively for Insomnia), so TGQ isn't going on that list regardless of its 10% or so greatness represented by Rando's input. Moreover, since all that content has been hosted on Insomnia, while no one else seems to give a fuck about it, I feel a certain justification in including it along with everything else in the number one spot with Insomnia.
Another site that would definitely be on the list is Nick Nasty's Segabastard, but this has utterly disappeared off the face of the internet, leaving practically nothing behind, and in order to determine its exact position I'd have to go through it more carefully than the casual glance I gave it before Nick took it down. I do have a correspondence with him, however, and he is considering bringing it back, or giving me the archive to host it on my servers, so we'll see about that.
Two other shoo-ins are gamengai and CAVE-STG.com (the latter of which by the way is closing down soon, so hopefully someone will manage to rescue the archive), but they will go further down the list because they are dedicated to very small domains of gaming (especially the latter), hence more general sites of comparable quality will have to take precedence.
So we basically have 8 sites so far. 4 more and we have a top 10. Perhaps we'll make it a top 20 after that -- we'll see. If you do post a suggestion, at any rate, make sure to include at least a few lines of justification, and one or two indicative links to articles or reviews.
Right off the bat let me tell you that HG101 will not be on it. And though sites like IGN and GameSpot were pretty damn neat in their early days circa 1999-2001 (I swear to God it's true -- look their early stuff up if you want), they have since degenerated to such a point that even mentioning them in a forum post is sacrilege towards the artform. So gimme what you got, and I'll keep racking my brains to come up with more contenders.