Re: Is there an area to run for the best gem drops?
Regarding the drop chance per monster, ghosts in nightmare and hell are best (apparitions, gloams, souls, storm casters etc.), but I guess at the time you can find and kill one ghost, you can kill more than 2 nightmare cows (which have about 1/2 of the drop chance of the top end ghosts) which are my recommendation if it's just about flawless gems. Also, Councils have a very high chance to drop flawless gems and it's always worth to visit 9 monsters of that kind if they are just next to a waypoint, no matter if on nightmare or on hell mode.
For more details, see the drop calculator of ATMA, the link is in my sig.