It's a strange question to ask, since I've played all the games except Arena.
I'm going out of my country as an exchange student, and I can't decide which RPG to have on my laptop (Win7).

I really want some cool diversity in choices for a build, for example make a full-on "Guardian of Peace", which will have a huge range of defensive abilities and Illusion spells for chilling everyone down. Or, a berzerker with super big damage but very low defensive stats. I want freedom for my creativity here.
As you can see it pretty much forces me to choose something from TES series.

But which one? Skyrim is not a variant, I don't think my laptop will run a modded Skyrim or even vanilla. (mods are neccesary for me here, as I feel like vanilla Skyrim is unable to satisfy my RP needs and it has many other problems that are fixed with mods).

I was thinking about Morrowind, since it's been a while since I've played it, and I remain a huge fan of this game. I remember great immersion, stories and unique detailed world. But the game has aged, and furthermore I've played sooo much, that I'm not sure if it will catch my attention as it did the first time. I need some feedback from you on it, to remember its greatness.

Oblivion. I've played it multiple times. And I know it is not as great as other games. But it had more interesting quests than Skyrim, and much more appealing atmoshpere than Skyrim's End of the World, Civil War, and grimmy gritty everything. Even though, ironically, it's exactly what was happening in Oblivion, too, hence the name. (Seriously they should stop with the saving of the world trope).

So, please, help me choose the best one for roleplaying and build diversity and interest, basing on your personal memory of it, or experience.

I also I would very much like if you could suggest the Must-Have mods for these games, because I need to install it all as soon as possible, and don't have time to browse through hundreds of them.

I'm more interested in functional mods than aesthetical, but I wouldn't mind simple little graphics mods like the one where you pick up plants and they change appearance.