The worst for me was the Minfernal so far. I didn't really camp it or anything but I kept coming to check for it from time to time. Finally I got it tonight, it was a miracle .
I also kept checking for snowy weather in Storm Peaks in the winter and it seemed to never snow there, but I got lucky one day and crossed the little fox from the list. I went to Dalaran and SW right away to let the others know in case they didn't have the pet yet, but no one seemed to care .
Between the two above, I still think the Minfernal was harder to get because it has a very long spawn time from what I herd and it's not dependent of weather or any other criterias that are easy to check (in my opinion). It also has a very small spawn area and the only thing you can do is survey the area from time to time to check for it, if it's not there then don't bother waiting, it's not your lucky day.
On a different matter - it was very awfull for me to get the green dragon mount dropped by the Oracle egg. My friends were very lucky and got it from the 2nd or 3rd egg, but I kept buying the stupi thing every week or so for almost 1 year untill it finally dropped.. it was really depressing and every time I bought the egg I said "this is really the last time I do this!" but we all know that's not true