Sid Meier's Civilization IV

No question my favourite and the best mainline Civilization game in the series - and a accumulation of all the good ideas generated from Civ 1 thru 3 (whilst culling quite a lot of the bad ones!) distilled into one excellent package. Subsequent sequels 5 and 6 do their own interesting thing, and I certainly admire attempts to mine new depths, but I have personal disagreements (mainly 1UPT to be fair) that prevent my from sinking too much time into them. Civ 4 ends up sitting in the perfect goldilocks zone for me.

Pretty much all of the overarching mechanics work well and are suprisingly un-wonky for how complex they all are (compare to Civ 3, where nonsensical game rules are strewn throughout), and the "Cities cost maintenance, Buildings are free" approach to limiting endless expansion should never have been abandoned in subsequent games. That combined with a serviceable AI (doomstacks, love 'em or hate 'em, do a lot to help AI's out with conquering), nicely transparent diplomacy and a really well constructed tech tree make this game still fun to play after thousands of hours into the game.

Its certainly not perfect, there are weak points/'one right plays' present (e.g. Slavery Civic being mandatory for high-level play, Great Lighthouse busted on high-water maps, mass tech trading if enabled etc.) and games can still be a slog to end due to some lacking automation and too-high victory requirements - but honestly? These complaints are not new to the 4X genre, and with the game old enough to have abundant modding effort poured into it, the game has never been in a better state than now.

Last Civ game done in the classic style (pre 1-unit-per-tile). It's mostly just incremental improvements on Civ1-3, but it was a lot of fun for me and I sunk tons of time into it. I still remember a lot of individual matches, all these years later. I don't really ever plan to go back and play it again, but it sure was a fun game.

There's quite a lot of mechanics to learn in Civilization IV, and so takes a while to understand the strategy. It's been a while since I've played Civilization Revolution and Civilization III, but I recall the general idea of forming cities, maintaining diplomacy, building an army, acquiring resources and expanding science and culture.

There is a "Tutorial" option which I think does a bit better than the previous game but you will need lots of practice to fully understand the mechanics. There is a "Civipedia" which has information on all the concepts in the game, so you will need to keep going to this to read it.

This is a turn based game and you move over a hex-based grid. Your city can access the surrounding tiles to acquire the resources, and the range expands as your population grows. You need Food for growth, Production to produce units, Money for maintenance, and your city can grow in Culture which expands your borders. The border is your national boundary, so only allies can pass through when you have the “Open Border” agreement. Otherwise, entering another Civilization’s borders declares war.

Tiles can be upgraded in some way such as building a road, mine, or irrigating it. You need to build lots of roads and make sure your towns are connected. Not only does this allow you to move troops quickly to help defend them, it allows you to share certain resources.

Although you would assume you will be fighting a lot, you are discouraged by the fact that your citizens can become happy and revolt; temporarily ceasing production. When you take over cities, the residents are partly from the original civilization, which means they will be unhappy whenever you are at war with their country. It seemed beneficial to steal a few towns, then offer a peace treaty, so you can grow in terms of the number of cities, but then allow your new cities to grow further.

When negotiating peace, the stronger you are, the more negotiating power you have. So you can get them to throw in extra gold, technology or even small cities. The AI will offer to trade with you when you are at peace, usually wanting your better technology. I normally liked keeping the upper hand here, but sometimes I accepted if I can trade multiple technologies for one.

When fighting, there's some random elements involved plus stats of the units and extra bonuses based on the tile. Some units are designed for attack, defence or bombard. When your unit is victorious, they acquire experience and can level up. Units in cities gain a defensive bonus, as do units that you instruct to Fortify, which over time, will give you a bonus up to 25%. When units occupy the same tile, attacking them automatically selects the best unit to defend (so if you attack with cavalry and there is a swordsman and a pikeman on the tile, you will attack the pikeman); so the aggressor never has the upper hand. These aspects mean it is really difficult to take cities.

You start off in BC, but as the turns progress, you move through the ages like medieval times, industrial, up to the present day. It's interesting to have basic warriors and catapults, then several hours later have infantry and tanks.
Notice that I wrote “several hours”. This game is a lot like the Total War series. So be prepared to spend more than 10 hours on a single game. There's lots of civilizations to take control of, but I'd imagine most people won't see many of them due to how long each game takes.

The definitive peak of the series IMO once you get the expansions. Deeply complex, great for single or multiplayer, and still with the original combat system that allowed for fast attacks and DOOMSTACKS. Now obviously doomstacks are a bit controversial in the community but I much prefer them to the half-baked hex-based combat that you get in V and VI. While the combat AI was never great, it at least still worked well enough in this to be a challenge, unlike in V and VI where one archer on a hill and a warrior can wipe out hundreds of enemy units that come in one at a time. Also some great voice acting and music complete a truly stellar game.