Where should you put your secret base in Pokemon sapphire?

I think the best place to put your secret base is by Fortree City on Route 119. Go straight down from Fortree City until you can sure on the lake. Head up the waterfall and you'll find some rare berries and planks standing in the water which you can only cross by using your Acro Bike. One you do so you'll see some mounds of grass on the other side. Test each one with Secret Power and choose your favorite!