Civ4.
1.) While the combat system feels entirely artificial (suicidal artillery charges while melee provides cover, general maths abuse, bonuses that look the same being handled in different ways...), it allows enough tactical depth if you make sense of it and
doesn't get in the way. You can rack up impressive kill ratios at tech parity if you know what you're doing, but a sane baseline strategy is easy to execute and the AI does an adequate job of it.
The Civ5 superimposes a truncated, awkwardly spaced and paced tactics game - all the hassle of that on top of the empire builder, a fraction of the depth. Executing a sane baseline strategy requires lots and lots of clicks and repeated double-checking... and most of the time, you're combating rampant idiocy rather than looking for an opportunity to show your brilliance.
2.) Civ4 is a rather decent empire building game. By BtS, it suffers a little from feature creep - many of the new mechanics feel tacked-on and poorly integrated but the game doesn't suffer much from it. Additional features balanced to be safe to ignore and requiring a lot more work to make work well... not elegant, but that's a safe way to increase player choices.
Civ5 needs to make considerable concessions to support its combat system - the game can't handle too many units, which is a huge constraint on balancing production. The original happiness model was useless to detrimental in preventing degenerate strategies. The gameplay of the release version gave the impression of not having been thought through let alone tested... unless the attitude was 'let's include as many marketable features as possible without considering how it plays'.
While it's good to see that patching continues (rather than just hawking DLC after DLC), I think there are too many fundamental design flaws to truly get anywhere.
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Incidentally: nice to see a really spiffy 'votes by location' map in the poll... of the USA only, broken up by states, without even an 'insignificant rest of the world' category.
Or am I missing something?