I read somewhere that the tongue contains the strongest muscle in the human body.
I just had an 'arm wrestle' between my tongue and my (left/weakest) index finger and the finger won (and it bloody hurt).
Think you might have had the finger on the wrong muscle, mate, but we won't go any further down that road for fear of offending the Victorian sensibilities of those who patrol this site at such a late hour.
It depends upon how your define and measure "strong" i you mean strength per area then the tongue must be the strongest.
If you mean "pulling power, then the Soleus in the back of the leg must have it.
If you mean muscular resistance to stretching, then the myometrium in the uterus must have it.