Most people here say that California Screamin's drops are not too bad, but as a drop-hater I found the drops and bunny hills, coupled with the adrenalin of the launch, made me rather shaky when I got off. I didn't enjoy it at all, and I can get through Splash Mountain OK and actually enjoyed Rock 'n' Roller Coaster. I would rank Screamin' as way worse than Splash Mountain, personally. If you do try California Screamin' I'd consider trying it at night, if you can; I've heard that helps.
I've never been on Grizzly River Rapids so I won't rank it, but it sounds like you got good advice on that already.
My ranking for overall DROPS (meaning the kind of rides where I feel like my stomach is traveling at a different speed than the rest of my body) would be:
Tower of Terror (never been on it because all it is is drops and cool effects and I only like one of those things

)
California Screamin'
Splash Mountain
Mulholland Madness ("wild mouse" coaster with lots of steep but short drops; also jerky; depending on your preferences you might even switch this with Splash Mountain)
Pirates of the Caribbean (I find the drops just the tiniest bit unpleasant sometimes)
Space Mountain (tiny bunny hills that I find delightful)
Big Thunder Mountain
Indiana Jones (same track as Dinosaur, right? technically something akin to a drop)
Matterhorn (no real drops that I recall, but super-bumpy so it's not my favorite)
Gadget's Go Coaster
The drops in Star Tours and Soarin' initially bothered me but as soon as I got used to the rides I was fine. You should know your tolerance on those from WDW though, right?
Have a great trip!