Firstly what would you say is the one single biggest and strongest piece of evidence in on favor of Islam?
Islam's main message which was the same message that has been preached by all previous prophets including Jesus(peace be upon him) is that God in one. So before one can first accept Islam is the religion revealed by the Creator to mankind one would first have to acknowledge the existence the Creator.
To do this one would have to reflect on creation.. Observe and study the world around us. I personally like to use the analogy of looking at something like a laptop. Simply by using reason you can figure out that from this laptop you can tell that there is a manufacturer. Then we can take apart this laptop and study how it works, then we would be able to use it to its fullest extent for our advantage. If anything, this should make us be even more awed by the design of the laptop.
"And He has subjected for you the night and day and the sun and moon, and the stars are subjected by His command. Indeed in that are signs for a people who reason. - Quran Sura Nahl 16:12"
Every verse in the Quran is called an "ayaah", which means "sign". And such is the nature of "signs", not everyone can see them. For example let's say we come across a snake with some interesting patterns on it's scales. Someone knowledgable would be able to tell that the snake is likely to be venomous while someone who's not knowledgable wouldn't think anything of it except maybe "Wow nice patterns."
From the Islamic perspective, the entire universe is a sign pointing towards the Creator. Those who do not reflect merely see the universe and admire it's beauty, then there are those who do reflect and can see that the universe did not come from nothing.
Secondly have you ever had any doubts about your faith? if so what were they and how did you overcome them (assuming you did overcome them of course)
There are very few people who have 100% faith. If our faith is at 100% then we would be sinless, which I don't think is a claim no one here would make. Doubts in faith however shouldn't necessarily be blamed on the religion(in this case Islam). A lot of times it is through our own ignorance, lack of knowledge as well as our lower desires(carnal?). So what is the best cure for doubt? Knowledge! This is why it's important to learn in Islam.
There is a Hadith where the Prophet (peace be upon him) said something like "Knowledge is the lost treasure of the believers, seize it wherever you find it.".
Another one is..
Abu Huraira reported: The Messenger of Allah, peace be upon him, said,
“Whoever travels a path in search of knowledge, Allah makes easy for him a path to Paradise.”
[Sunan At-Tirmidhi, Book 39, Number 2646,
Hasan]
That said, even in learning there are proper channels/etiquettes. Not only would one need proper teachers but more importantly the proper intentions, and also humility. But that is a whole other subject already.