In my opinion the biggest spawn of only wyverns is on Edron.
In terms of wyverns + other minions I think the biggest are on warzone 1-3 task (probably gnomish gardens? Correct me if i'm wrong with whiplers and eartch elementals).
Also You can spot wyverns during making medusa oinament task and I think that might be the best spot - long time since last time I was doing this task so You need to check it by Yourself.