From the ATMA program (check single player forum for more info):
In a single-person game with 0 MF on:
Nightmare Baal: 1 in ~2887
Nightmare Diablo: 1 in ~2902
Hell Mephisto: 1 in ~3264
Hell Diablo: 1 in ~3390
Hell Baal: 1 in ~3437
In a four-person game with 400 MF on:
Nightmare Baal: 1 in ~994
Nightmare Diablo: 1 in ~1000
Hell Mephisto: 1 in ~1125
Hell Diablo: 1 in ~1167
Hell Baal: 1 in ~1184
So, IMO, Nightmare Diablo is the best to run for an Occy unless you have an unmentionable outside program.
If you do have a program like this (Again, don't ask for one here) then I'd imagine Mephisto would be better, but only because the Durance of Hate level 3 is freakin' humongous in Nightmare and Hell in version 1.10 and can be hard to navigate through legitimatly.
Hell Meph shouldn't be hard to find. Teleport clockwise and you will end up getting there in max 30 seconds if the stairs is long away from the wp. Don't really see the problem compared to Diablo where you have to open seals and jadda jadda.
Mephisto is good in other ways too since you can find other great stuff you can trade for an Oculus and generally improve your gear :thumbsup:
occy's just easy to find...nitemare meph is prolyl easiest place to find it...just cuz he's so damn easy! and price..well...if ur lucky some guy'll throw it away...other wise as someone before said...lem-pul