Hmm. Honestly foliage is my favorite part. It comes naturally to me placing trees and shrubs everywhere.
To start, stick with StationJim's sets. If you use all of those you'll be fine. For special things, though I use a few other sets.
Generic - Pretty much any tall trees from N7's packs for the forests, and in the actual park you want some less obvious foliage. Your job is to make it look good, not to tell everyone "Hey guys, look. Flowers! Flowers everywhere!". Put a couple of
planters in the middle of a wide pathway to add some decoration around the dull concrete. For the actual rides, I suggest using some kind of theme, even if it's not over the top. Along with every theme, however, comes different styles of foliage.
Tropical - I mostly use Spice's Palms V1 and V2. Also a little Weber's Foliage and Jungle Jumble in the heavily wooded areas.
Desert - I haven't done much of that, but I would probably use a few cacti and extra small plants/weeds here and there. Unless this is Dubai you won't need much greenery.
Spooky - Since this theme is typically presented at night, foliage is not as important to get right as far as colors and looks go. You can't see well in the dark. Lots of people just use generic foliage and when you add some magical twinkle fairies and smoke on the ground, it really compliments the spooky architecture.
It's actually not as hard as it seems to create good looking foliage. One thing I've noticed that you are doing is using the same set or even the same type of plants for large groups of flowers. If you really want to get it looking awesome, use at least 3 different sets of plants and it turns out pretty good.
Also, make sure you look at other examples. Whether it be real life examples or other RCT3 parks, you can look at those to help you know where and where not to put stuff.