I think it is weird what people have heard about all thee MOOs. Although it might sound like I think I'm right (I do), this is based on 10-15 years of experience with the series.
MOO: One of the best strategy games ever, and computer games ever. Unbelievably good replay value and fun. As good as Civ4.
MOO2: Very good game, falls short of the original MOO because of the loss of one of the dimensions of the original game, which is that NUMBERS matter. Your ship design might be great but HOW MANY you have is almost more important. Also, that plays into the longer running strategy game of aging technology and how useful it can be, which can be seriously affected by numbers. Same for other aspects of the game, where there is immense strategy surrounding all things related to HOW MUCH including population, factories, etc. The scale of your operations is critical and this aspect was lost in MOO2 and replaced with nothing, making it less deep or engaging. Its a good game, just not as good.
MOO3: Never even contemplate loading it, it would be sub par for a crappy strategy game. Its insulting next to MOO2 and is an outright laughable disgrace of a failure verses the mastery of the original MOO. It shouldn't have ever even shared the same name. NOT worth even considering.