Formal:
* Mo ni ayọ̀ pupọ̀ (Mo ni ayọ pupọ) - This is the most literal translation and means "I have great joy."
* Inú mi dùn pupọ̀ (Inu mi dun pupọ) - This translates to "My heart is very happy."
Informal:
* Mo dùn gan-an (Mo dun gan-an) - This means "I am very happy" in a more casual way.
* Mo gbadun gan-an (Mo gbadun gan-an) - This means "I am enjoying myself very much."
You can also use expressions like:
* Mo fẹ́rẹ̀ẹ́ gbàgbà (Mo fere gbàgbà) - This means "I am almost bursting with joy."
* Ayọ̀ mi kò ní òpin (Ayọ mi kò ní òpin) - This means "My happiness has no end."
The best way to say it depends on the context and the level of formality you want to convey.