Civ 3 was, beyond a doubt, the worst of the series, in my opinion. Civ 3 was like doing your taxes. Some autistic people really get into that (doing taxes and Civ 3, both), so I guess there's always going to be some people who love Civ 3 to death.
Civ 4 was basically Alpha Centauri with better graphics and AI, and I loved Alpha Centauri. So, I really liked Civ 4, on release. There were lots of issues, but it was a pretty good release. I seriously disagreed with the 9.95/10 reviews that came streaming in (I thought it was more like an 8.5 or 9.0/10), but what can you do? Game review sites are basically just another cog in the marketing machine. They're a complete joke. Also, the fanboys were highly annoying. Anyone else remember the "thank you. it's perfect." thread that sprang up? Ugh.
So, I'd definitely say that Civ 4 was better on release. Civ 3 was an abomination. Yeah, conquests made it better, but it was still kind of like doing taxes, while you balanced your checkbook, and did math homework. Civ 4 was like composing a symphony. Civ 5 is... is... hmm... I'll have to get back to you on that. Preliminary tests reveal it to be more akin to riding a bicycle with training wheels, but, hey, not everyone wants or needs the complexity of a 10 speed bike. Sometimes you really do want a game with training wheels. It's not a put-down, necessarily.