They are Shaymin, Manaphy, Mew, Victini, Celebi and Jirachi. (Gen 7 OU)
If we go by SM OU viability rankings, Victini and Manaphy sit in the highest tier among the options you listed
This outlines why Victini is good, particularly V-create allowing it to be a good band/scarf user and Z-celebrate allowing it catch unknowing teams off guard with a sweep.
Manaphy had a strong niche in rain teams thanks to its natural bulk and hydration letting it stay healthy in the face of status and Tail glow letting it get large amounts of Special attack.