Chimney leak

I suspect there is a leak at the chimney/ roof joint. What is the best material to seal this with? Appreciate the help!

Sometimes I use a product called Blackjack, you can find it at Lowes, Home Depot, etc.. It's very messy so be careful and wear gloves. You can use a disposable plastic trowel to apply.

Or...... This product works very well too. Found at the same locations. Your call.