I found the Battle of Pelennor Fields (Free Play) to be the easiest way to get studs.
1) Turn on all the multipliers (of course) and the Attract Studs brick.
2) Start Battle of Pelennor Fields.
3) Set down controller - your character will ride through the first part automatically.
4) Pick up controller when Fell Beast attacks and pick up the three purple studs and their neighbors.
5) Save and quit, 235,000,000 studs richer in under two minutes.
Note: You can make 275,000,000 studs if you manually control the rider in the first part and grab a purple stud that is hidden in a bush just before the ramp - probably 2/3 to 3/4 of the way through the ride.