This is something I looked for while going for the 70 Gold Medals achievements, just a list of someone who had completed it of the missions they though were the easiest so I thought I'd post the 70 I completed in case someone else would benefit from it. Obviously this list will differ depending on play styles but hey it could be a start.
Prologue, Franklin and Lamar, Repossession, Complications, Chop, Father/Son, Marriage Counseling, Daddy's Little Girl, Friend Request, The Good Husband, Casing the Jewel Store, Bugstars Equipment, BZ Gas Grenades, The Jewel Store Job, Trevor Philips Industries, Did Somebody Say Yoga?, By the Book, Hotel Assassination, The Multi Target Assassination, Cargobob, Tow Truck, Boiler Suits, Masks, Trash Truck, I Fought the Law..., Eye in the Sky, Mr. Richards, Paleto Score Setup, Hang Ten, Reuniting the Family, Doting Dad, The Ballad of Rocco, Meltdown, Parenting 101, Gauntlet- Mission Row, Gauntlet- Rockford Hills, Gauntlet- Pillbox Hill, Something Sensible, The Time's Come, The Vice Assassination.
Strangers and Freaks
Pulling Favors, Shift Work, Paparazzo, Grass Roots- Michael, Rampage One, Rampage Two, Rampage Three, Rampage Four, Rampage Five, Grass Roots- Trevor, Exercising Demons- Michael, Paparazzo- The Sex Tape, Vinewood Souvenirs- Kerry, Pulling Another Favor, The Civil Border Patrol, Risk Assessment, Vinewood Souvenirs- Tyler, Vinewood Souvenirs- Willie, Minute Man Blues, Paparazzo- The Highness, Liquidity Risk, Paparazzo- Reality Check, Targeted Risk, Pulling Favors Again, Still Pulling Favors, Exercising Demons- Franklin, Pulling One Last Favor, Uncalculated Risk, Exercising Demons- Trevor, Closing The Deal, Surreal Estate.
Hope all that typing at least helps somebody, and gives a little bit of an idea towards the easiest/ fastest missions I found.