The Obvious approach for The Big Score requires you to have finished both Sidetracked and Driller. Once you do, a "H" will appear on your map, which will trigger the beginning of this mission. Make sure you have gotten armor for your characters, as well as get some ammo for them all.
You start out playing as Michael, do get in the car and head to the bank, where the game will switch to Franklin. Drive the cutter to the marker on your map and drill a hole through the wall and into the bank vault. Once you do that, park it out of the way, then go inside the vault and blow open the metal doors.
As soon as you blow open the doors and exit the vault, cops will start pouring in from the tunnel nearby. Use cover and kill them as they appear, with more showing up on the other side when the first wave is finished. Once you clear the second wave, run around and wait for the scene to hook up the gold.
When this happens, more cops will appear behind the rubble inside of the vault, so kill them all and then Trevor will mention that some NOOSE are rappelling into the tunnel. Head back out of the vault and kill them before they can land. Switch to Trevor now and fly out of the city until you get told to switch to Michael.
You'll be on foot and will have to fight through A LOT of cops and NOOSE agents, so start by taking out the ones in front of you. Get to the end of the initial area and a helicopter will appear. Feel free to take it out, then head down the stairs to meet up with Franklin. You can switch to him now if you want, but stick to Michael because of his ability, only switching if he's in danger.
Pick off some of the enemies from the ledge above, then head down the stairs and cross the bridge. There will be another helicopter that appears, so bring it down and then continue to the other side. Go to the left, but don't go down the stairs. Take cover behind the railing and take out the chopper that shows up, then go straight and up the stairs ahead.
There will be yet another helicopter flying in, so hide behind the stairs and get a clear shot at it before going up all the way. Now you can head up and take down the rest of the guys, then head to the other side of the building. What else is there to do, but destroy another helicopter here, one which can fire missiles.
Follow the others, fighting your way through more enemies, until you get to the parking lot underneath the building. Fight off the few enemy vehicles that appear, then get in your getaway vehicle. You need to escape a 5-star wanted level, which is easier than it sounds, especially if you stick to the normal streets.
However, there is an easy way to get rid of the wanted level. Go left when you get in the car, then turn right outside of the garage. You should see the bridge from The Jewel Store Job to your right, so head off it to the highway below and then go inside the tunnel area. Here, just drive until you clear the wanted level, which will make you switch to Trevor.
You'll have to deal with three Merryweather helicopters, who are coming after you, but you have to contend with flying the chopper. In fact, it will be Lester who will have to take down the enemy helicopters with a RPG. What you need to do is fly close enough to them and then hold it steady to let Lester get a clean shot.
Once all three are downed, fly to the train and get the gold onto the car. Land at the airfield to finally finish the mission.