I’ve never got what I consider ‘strong coffee’ from a K-cup.
Closest is Death Wish K-cups, but not really worth the money for K-cups. By contrast, regular brewed Death Wish is DY-NO-MITE.
It takes some trial and error to find strong K-cups. Some coffees that are strong in ground or bean form are not strong in K-cup form. Also, some machines have different settings. Our home unit has a “strong coffee” setting which runs slower and steeps the coffee longer. Also, one can vary the cup size. I usually set mine for 8 oz; my wife prefers 10 oz. The Peet’s that I like is pretty strong. My wife likes the Costco Pacific Bold. Any nationally known brand like Folger’s will be pretty weak in K-cups.