Of all the classes throughout the games, which do you guys think is the strongest? This can be lore or gameplay reasons and try avoiding creating a custom class version, of a class.

Gameplay-wise it differs with each game.

Mages were the most powerful in Morrowind, once they reached a decent level. They could do everything the other classes could, and more.

Mages/Assassins were pretty close in Oblivion, so a mix of both would be the one. Nightblade, I guess?

Assassin in Skyrim.

Note that the above is from personal experience only.

Lore-wise, it's not possible to pinpoint which class would be strongest since that has to take into account the individual. It would probably be a Mage again though.

/heavy bias to the mage class

I'd have to say Battlemages both gameplay and lorewise.

Mage and Battlemage in lore and depending on game Morrowind: Mage, Oblivion: Battlemage and Nightblade, and Skyrim: "Illusion Assassin" (no classes but I guess closest to Nightblade)

I would say spellsword/battlemage type characters in lore, and honestly a pure warrior in gameplay. At least I have the easiest time in all the games playing a pure warrior.

Mages. Even in the Arena manual it was stated as such.

In terms of gameplay that changed by Skyrim, where stealth classes are better.

Lorewise, I'd say mages again, but I am also heavily biased.

I'd also say Illusion is the most powerful in-game skill a regular mortal can obtain, the influence to manipulate and bend the will of the minds of others is to me above all others in these games and TES in general, especially in such a political environment as Tamriel.