Rollercoaster Tycoon was the best IMO
there's also Pizza Syndicate, which is called Fast Food Tycoon in some places, that one is also good, but nowhere near RCT.
I've never played Zoo Tycoon though.
edit: how could I forget Railroad Tycoon?! Railroad Tycoon 2 is awesome! there's also Transport Tycoon but I didn't really play that one, only OpenTTD, but not as much as I played RCT and RT2.
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