So speaking as an ex-JW how I would've answered when I was a JW
1. What is the most fundamental reason you do not accept the Roman Catholic Church to be the one true institution of God?
Because they pray to Mary which the bible strictly tells you not to pray to anyone but God. They also make graven images of their God, they use a cross in their worship which is a pagan practice. Finally the problem with Child Molestation in the Catholic Church using the scripture that "By their fruits you will know them".
2. What is the most fundamental reason that you accept your organization to be the one true organization of God?
When I was a JW I guess the answer would be that they were the only ones who would change their religion as they learned truth instead of sticking with their proven false teachings. They only worshipped God, they didn't worship any other deities, they didn't take part in any pagan traditions and the followers knew their bible far better than any Catholic.
Now I know that essentially everything that applies to Catholics also applies to JWs, which in turn applies to all religions. Hence why I'm an atheist.