We should define Kekkei Genkai first, because Hashirama's Wood Style is, in my opinion, not a Kekkei Genkai but a self-developed ability. He could use both Suiton and Doton and ended up creating Mokuton. That's my theory, since no other Senju was shown to use that ability.
Mu taught Onoki Dust Release and Third Raikage taught Darui Black Lightning. Kekkei Genkais aren't inherited like that, you need to be blood relatives.
With that said, I'd say the strongest is Minato and if we consider Wood Release -NOT- a kekkei genkai it's obviously Hashirama.