I have started collecting farming data on Goblins, in order to better track down Rainbow Goblins. Over the last 10 days I have run 32 different maps 10 times each, doing full clears until I find a goblin or run out of places to look. I'm posting my results to help people decide which maps to run, but keep in mind that clear times are the important thing, and we are all going to have different clear times.
I chose maps that I've heard people talk about in goblin farming communities, Both good and bad.
1. Festering Woods - 11/100 (1 Rainbow Goblin)
2. Cathedral 1 - 31/100 (1 Rainbow Goblin)
3. Leoric's Passage - 30/100
4. Cathedral 2 - 14/100
5. Cathedral 3 - 16/100
6. Cathedral 4 - 21/100
7. Royal Crypts - 17/100
8. Southern Highlands - 9/100
9. Cave of the Moon Clan Level 1 - 25/100
10. Cave of the Moon Clan Level 2 - 29/100 (2 Rainbow Goblins)
11. Northern Highlands - 48/100
12. Leoric's Hunting Ground - 26/100
13. Fields of Misery - 0/100
14. Howling Plateau - 36/100 (1 Rainbow Goblin)
15. Stinging Winds - 21/100
16. Ancient Waterways West Level 1 - 13/100 (1 Rainbow Goblin)
17. Ancient Waterways West Level 2 - 19/100
18. Ancient Waterways East Level 1 - 16/100
19. Ancient Waterways East Level 2 - 17/100 (1 Rainbow Goblin)
20. Dahlgur Oasis - 0/100
21. The Battlefields - 18/100
22. The Fields of Slaughter (Bridge of Korsikk Waypoint) - 17/100
*Pandemonium Fortress Level 1 had 23 Goblin Encounters, for a total of 46 Goblins. This level contains 0 or 2 Goblins.
Total Rainbow Goblin Encounters = 9
Total Goblin Encounters = 651
Please feel free to test out maps and post results as well, 100 is a fairly small sample size and I'd love for more people to collect goblin data.
The next step in the process of determining which routes to take, or which single map to run, would come down to clear times. Clear times are basically from Clicking "Start Game" until the next time you can click "Start Game." Loading in AND out is part of clear times and has a huge impact on single map runs! I'd love to start posting average times of different class' farming setups to help people pick a class and route... but that will be a project for the future.
Goblins are most common on these individual maps: Tower of the Damned, Core of Arreat, Gardens of Hope Level 1, Cathedral Level 1, Leoric's Passage, Northern Highlands, and Pandemonium Level 1.
Goblins are also fairly common in these multi-level maps: Ancient Waterways, Rakkis Crossing + Underbridge, Cave of the Moon Clan.
The frequency of encountering goblins does matter, but only when balanced with clear times.