Civ V
TBH the combat overhaul is Civ 5 is the best thing to happen to the series since... well, ever. It's FAR more tactical now than it ever was - you no longer just create an enormous stack and go yomping but create a mix of long and short range units and use terrain (such as hills - or even mountains once you've got helicopter gunships) to your advantage. That, in itself, makes Civ 5 probably my favourite Civ game yet (and I've been playing it since the first version on an Amiga A500+).
As an added bonus, the annoying espionage system from Civ 4 has been dropped.
There are a few things I miss from Civ 4 however:
1: The United Nations Resolutions - the UN is now just a crappy win mode for whoever bribes the most city states.
2: Actual benefits from researching "Future Tech" - at the very least I'd love it to up the happiness of your civilisation slightly; the global happiness thing in Civ 5 gets a bit naff at the end-game when you've got lots of huge cities - you have to adopt some very weird cultural policies just to keep your civilisation happy; e.g. Honor (the "early game" combat policy) to get the happiness bonus from each "city wall" upgrade (walls, castles and so on).
3: Religions - they're just not in Civ 5 at all, which is a bit of a shame because they worked well in Civ 4.
4: Different leaders with different bonuses per-Civ; just one leader per Civ atm and therefore one set of predefined bonuses per Civ.
... still, many of the big changes in Civ 4 that made it the Civ 4 we all knew and loved came in with the 2 expansion packs. The way things are going with Civ 5 there'll not be any expansion packs just a continual trickle of DLC which may eventually rectify any/all of these little gripes *shrugs*