I own Rollercoaster Tycoon 3 but haven't played it much in the last few years. Since there's a sale on the series today, I wanted to know if any of the previous versions were as good or better. I don't care if the older ones were in 2D, just fun and engaging. Thanks in advance for any tips!

Both Rollercoaster Tycoon 1 & 2 were great, and were some of my most played games when I was a kid.
They are largely very similar to each other, with RCT2 having minor improvements to things like the Guests path finding, and being able to decorate your rides a bit more.
I couldn't really recommend one over the other as they both have scenarios that I love playing to this day.

Lottsa fun to make and create the winning strategy to running a successful theme park in the original Rollercoaster Tycoon (Deluxe).

I never got into RCT3 as much as I got into parts 1 and 2. It's easier to move around and manage everything, because it's a raster-based playground like many older Sim games (e.g. SimCity). It also has this very charming pixel style and some of the most memorable music. I don't know whether you will have the same appreciation for it if you played RCT3 first.

I think RCT2 is more like an add-on to RCT1 as a self-standing game (even though that is not the case). It has the same mechanics, interface, graphics, everything. It's really the same game, but with more features. Knowing that can affect your appreciation of these games. I found it difficult to go back to RCT1, because RCT2 had many convenient features that allowed for a much deeper gameplay. I am the type who uses the rides and landscape theming to the full extent and that's where RCT2 is a lot better.

FYI, you can add very accurate renditions of RCT1 scenarios in RCT2, along with it's own fanmade expansion. So that pretty much whittles it down if you're looking for sheer content between the two.

I never got the hang of 3 so much. There was just something about it that felt off to me.