Ningaloo Reef, Western Australia!!
Coral Bay has manta rays all year round and it is extremely rare to go on a manta tour and not see them.
Exmouth has mantas mostly from June through November though there are some around all the time, it is more unusual to encounter them outside of that time frame.
Mantas are often encountered on Whale Shark tours, too (late March through into July most years). so you might like to consider seeing two of the big guys in one trip as it is so very far away for most people
Coral Bay has manta rays all year round and it is extremely rare to go on a manta tour and not see them.
Exmouth has mantas mostly from June through November though there are some around all the time, it is more unusual to encounter them outside of that time frame.
Mantas are often encountered on Whale Shark tours, too (late March through into July most years). so you might like to consider seeing two of the big guys in one trip as it is so very far away for most people