I haven't really fully thought out my tier list until now, so this will be fun. The dividing line between A and S ranks is going to be particularly difficult...
This is also probably going to be biased towards King's Island and Cedar Point, since those are the parks I've been to most recently.
Definitely S-Tier
Millenium Force (Cedar Point) is definitely an S-Tier for me. It's an Intamin gigacoaster with an interesting lift mechanism. No inversions (which is a plus for some!), but a huge drop at the beginning, a lot of speed, and a lot of airtime.
Diamondback (Kings Island) is also on the list. It's a B&M hypercoaster with a splashdown at the end. Another good one for airtime lovers, and it offers amazing views of the park. If you can get the timing right, it's a great ride to be on for the end-of-day fireworks show.
Another one that's up there at Kings Island is Orion. This is another B&M coaster, one that seems similar to slightly-scaled-down Fury 325? Other manufacturers would classify this as a gigacoaster since it's got a 300ft drop, but I think B&M calls it a hypercoaster. It's got a nice large drop, and it would have a very good view of the park if it weren't located right on the edge of it. Good height, good speed, good airtime.
Hm... this list is suspiciously lacking in inversions. Let's fix that with Valravn (Cedar Point)! Another B&M coaster, this one has 3 inversions, and it holds you over the 90 degree drop for a couple of seconds before releasing you. I love that feeling of suspense.
Finally, we've got Banshee (Kings Island). This is a B&M inverted coaster, and it's got 7 inversions, one of which is a loop that wraps around the lift hill. If you love loops and flips, you're going to like this ride.
Now we move on to the more-questionable entries
Let's start with Mystic Timbers (Kings Island). It's a 2017 wooden coaster, and y'all... this is the smoothest wooden coaster I've ever been on. I tend to to think of wooden coasters as "potentially fun, but it'll probably rattle your brain", and this ride was magical. Also! Also! Theming!! It's got theming! More specifically, "Haunted Lumber Mill" theming. Pretty well done, tbh, especially for Cedar Fairs. The reason I'm not sure if this is S-Tier is because it's hard to tell if I like it that much, or if I'm just so surprised by how smooth this ride was.
Next up is Steel Vengenace (Cedar Point). This was the first RMC coaster I had been on, and WOW these guys know how to make a goddamn coaster. This ride has dynamics. They have special lockers for you to put your phone and wallet into, because this thing WILL empty your pockets if hadn't already. It's a fun ride. Literally the only reason this isn't in my S-Tier is because I only got to ride it once, and I want to be sure it's jsut as amazing the second time.
Now we're onto the "It's been so long and I was young enough that I don't clearly remember, but I'm pretty sure these were good" section.
To start us off, we've got Griffon (Busch Gardens Williamsburg). It's a floorless Dive Coaster designed by B&M. Same start as Valravn, where they hold you over the drop for a few seconds. Also got a splashdown element at the end. I just don't remmeber this one clearly enough.
Next is Apollo's Chariot (Busch Gardens Williamsburg). B&M hypercoaster, lots of airtime. Famously, during it's first run, the model Fabio was on it, and he was struck by a goose! Again, it's been so long since I've gone to this park that I can't remember how much I enjoyed the ride.
Currently got another Kings Island trip planned, and I'm thinking of trying to stop by Kentucky Kingdom in the next couple of weeks. The last time I was there it was still a Six Flags park, and I think almost all of the coasters there have been removed or renovated since then. And they've got an RMC coaster now! I'm also looking forward to Coaster Con 46, which will be my first Coaster Con. This list could very-well get an update soon.
Anyway, this post has started to get away from me, but that's my S-tier list right now!