What's the order of quality in the Civ series and what makes it that way?
4 = 5 >>> 3 > 2 > 1
Some would argue that 2 and 3 should be switched. I think the more modern UI and accessibility make 3 better, but if you played 2 when it came out you might like it more.
4 and 5 are the really good ones, and they succeed in different ways. 4 is like a culmination of what made the series good. Everything is well thought out and they've gotten rid of some dumb stuff that plagued the earlier games, but it's still like the first few.
5 is something of a departure for the series, in some good ways and some bad ways. 5 has by far the most interesting combat system of the series - the last few games were known for their "stacks of doom" where you stack 70 tanks and mass move them onto a city as standard practice. 5 makes armies more expensive and limits to one unit per tile which makes combat a lot better and less of a messy cluster. 5 is also a lot more streamlined than the rest of the series.
5 has a less-developed diplomacy system though - if there's ever an expansion, we can hope they address that. If not, it's still a good game, though one that lacks a couple areas of depth that 4 had.