Information on RCT Classic: https://rct.fandom.com/wiki/RollerCoaster_Tycoon_Classic
Really, its just a different way to play RCT2 and includes most content from RCT1 and expansions.
If you want the most complete experience you'd need RCT2 + TT + WW and RCT1 + CF + LL. Without RCT1 you're mainly just missing out on those scenarios, and I think you can now have transparent water like in RCT1 in OpenRCT2 if you have those games. (There may be other small things, I'm not sure).