Oddly, I am never really satisfied with a 2 hour session or less playing an RPG. I love to tell an epic story and it’s rare that you can get one told in 2 hours or less. But I have done it and still had fun. In most cases, it was done at a convention as a demo for a particular system or a quick arena fight. The “best” system depends on your audience and what you intend to do in those 2 hours, for me. So my answer initially is “Depends!”
For a quick one-shot with a little story and one scene of combat, I would probably go with either some form of d6 system if the session os more or less improved or Call of Cthulhu 7e if there is some structure to it. d6 system is the easiest to explain and get started in. Call of Cthulhu 7e has some nuances that if built into the structure of the adventure (like in Necropolis) can be easy to get players through. Either is great for players that are familiar with it either systems. Additionally, if you have a veteran group of players familiar with the system, Savage Worlds is a perfect for quick 2 hour games. But there are some nuances in Savage Worlds that I would be nervous to introduce to new players if you have a time limit of 2 hours or less.
For a quick arena fight, most any system will do if you have a group of people familiar with the game system. We used to do this for Champions in Hero System. Build a character that day and we all fight in an arena. Character generation took forever but the fights were fairly quick. Again, Savage Worlds would be good for this too.