Several ways to go for deciduous trees...
If you're goal is very-good looking trees, then I'd consider SuperTrees. The $100 package that is offered by Scenic Express will make a lot of trees.
You can amp-up the look of SuperTrees by mixing in some of the other brands like Woodland Scenics (using polyfiber to represent foliage and not their Clump Foliage), JTT, Bachmann and Woodland Scenics more professional trees. A problem with SuperTrees is that the trunks are pretty spindley... Even by N-scale standards. By mixing in other brands you'll get some trees with more realistic trunk diameters, plus you'll have some trees that simply look different than that SuperTrees– And creating a contrast is good.
For hillsides, many still use the old-standy– polyfiber balls. Roll'em, spray'em with hairspray & finish them off with your choice of ground foam. The challenge with polyfiber balls is that it gives a pretty generic look– not much created in the way of contrast.
Then there are some outside plants and weeds that can be finished off to create a tree. Quite often these do not have the right shape to represent a front-line tree, but could be used to create a forest canopy.
I've found that by investing some time & effort into your trees, it really improves the overall look of the layout.