In my opinion the best way to become a good solid line pilot is to fly the line. From my experience, people who have plodded around in a C152 for years beforehand have no better performance than cadets.
There is no room for this 'working class hero' crap we hear about how you should bash circuits for hours or fly some clapped out old taildragger if you want to be a REAL pilot. If you want to be an airline pilot then get there as quickly as possible. A year flying the line will meake you an infinitely better 'operator' then crop spraying ever would do.