It's a hypothetical question, with the following stipulations:
1. The campaign you'll be running will be at least 60 sessions long, more if you wish.
2. It's a fantasy campaign but the subgenre is whatever you want it to be. Heroic, grim-dark, military, whatever.
3. Can be any rpg system since the beginning of the hobby, out of print or currently published and you and your players have access to the materials.
4. Putting aside character death and character retirement in the course of playing, the intention is to run with the same characters for the entire campaign.
5. You have adequate time to read rules and prepare. Miraculous, i know... but for this hypothetical, you don't need to worry about that. You have the time.
6. Also don't worry about getting players - let's say you have them and they want to play and will learn the game if they don't already know it.
7. Let's say there's good online VTT support for whatever system you choose. I know, not realistic, but it's not a factor you need to consider if you would run this online.
So that's it. What system would you choose for a fantasy campaign of considerable duration given the stipulations above?