Here's my 2¢ for what it's worth...
(1) If going strictly by simple mechanics and ease of use and understanding, you can't go wrong with Index Card RPG and Tiny Dungeon. EZD6 also, now that I'm thinking about it. All three are very simple mechanically. Especially Tiny Dungeon. For a new GM these would be very first timer friendly. For new players to, for that matter.
The down side is that there aren't a ton of prewritten adventures out there for them. There's some, but not many.
Index Card has a free Quick Start on DriveThru. Not sure about Tiny Dungeon and EZD6. I'd have to check.
(2) Can I get some love for Tunnels and Trolls on here? Maybe not the best system for a straight up newbie GM, but a GM already with experience as a player in other systems shouldn't have much of a problem. There's a tad bit of math involved but after a combat round or two it'll become second nature. It's pretty easy to pick up. There's a shit ton of solo adventures out there for it. Might have to do a little more digging for GM led adventures but they're out there. Shouldn't be a problem.
A Quick Start or free rules? I'd have to check.
Tunnels and Trolls is the shit. Don't know why this game doesn't get more love than it does.