Imam Khomeini ® told that there is only one du'a that all the imams (a) have been said to recite.
It is du'a Shabaniyah: http://www.duas.org/invocation.htm
Don't think it's just for Shaban... It's a great du'a...
Beside if anyone needs a really strong du'a you can ask someone better than urself to make it.
(eg. Imam Zaman AJ, Fatimah Zahra AS, Imam Ali AS, ...)
The dua that all 12 Imams AS recited is The Whispered Prayer (Munajat) of Shabaniyah.