[Taqwus]

It's not just the gem income that's important, remember. High-level, rare sites don't necessary provide oodles of gems, but may allow recruitment of interesting troops (ethereal knights, ethereal assassins, powerful mages, etc), or even free summons (mostly undead and devils), or other services (boosting experience, scrying) or bonuses (20% discounts on Conjuration, Construction, etc). Spiffy stuff out there if you're lucky.
Oh, and try not to get addicted to Acashic Record. It's expensive to cast...