Which RPGs are the easiest for you to run?
The ones where everyone at the table already know the rules. I prefer to focus on the story going on and I've at various times defaulted to the players for a rule. I make them look up a rule as I keep the game moving forward. I am actually okay with letting a player know the rules better than myself.
That said, I prefer the "lighter" rule systems that don't have a rule, or subsystem, for every type of action. I am okay with a system that is crunch heavy or math-intensive at character generation as long as once play starts things can move fast without the heavy crunch or rules.
Right now, that means 5E D&D. Most of my players know the rules at some level, even if it is just knowing previous editions. It is a fast system (at least at 8th level). The Advantage/Disadvantage system is beautiful for making fast judgements on bonuses to a die roll ("that awesome speech earned you advantage on persuading the guard to let you pass") because it comes down a simple 'did your action gain you an advantage on the task at hand? Yes or no.'. No more is there the question of 'does that action give a +2 or +3 or +5 bonus?'. Between al those points, right now, it the easiest for me to run.