Carrots and sugar cane are great farms (if you don't mind actually doing stuff) as you can use fortune on them without your tools breaking (although replanting carrots can be tedious if you don't have a good technique.
An afk fall trap with level 1 pig spawners is pretty much as good as it gets, especially if you add a lava layer into the drop (for reference, 1 pig spawner gives 200k per hour, and takes 12 hours to turn a profit).
When you have about 50-60 pig spawners, you should start making a creeper fall trap (for reference, 1 creeper spawner gives 720k per hour, but takes over 2 days to turn a profit).
Avoid witch spawners at all costs - sticks, sugar and glass bottles clog up your hopper systems,lowstone sells for next to nothing, and the gunpowder drops are so miniscule that even a cow farm makes more.