BI, however I still miss feats from both STW/MI and MTW/VI. But, as kekvitirae stated I´ll prolly never go back to any kind of board game. RTW/BI´s uber strat-map, the skins and other graphics tilts my vote. BI´s seemingly better strat- and battlefield AI makes me vote BI.

Strat-map:
IMHO one of the most annoying things in RTW/BI is that I can´t see all the land I control and to do it I have to build watchtowers all over, quite unrealistic. It´s annoying that I can´t raise WT´s in my own land. Movement rates are another big issue and build´n´recruiting times. Pathfinding is a mess.

Faction feeling:
I also feel that when you play the Romans (any) you feel that you are Roman but any other faction doesn´t give you that feeling. I´ll like to see more cultural "feel" of each faction. I like to se CA putting equal amount of work into EACH faction of the game.

Strt-map AI:
One of the major flaws is the constant strat-map AI re-evaluations that goes on. The AI never stick with a target/ goal and the result is that they run around aimlessly.

Btt-map AI:
I dream of the day an AI-army will hold formations, stick with a good defensive position and actually counter my moves. Ex. The spread out default formation in two lines works to the AI´s disadvantage every time. The AI can easily be pushed off a hill by simply outflanking. The AI will run a HA to the edge of the map trying to outflank you and it gets stuck there since it simply don´t "understand" that it can´t go any further. Javs/ missiles charges your inf before that have depleted their missiles. Phalanxes, in formation, keep chasing cavs and HA´s etc etc.

Personally I like realism and micromanagement and I´m a "builder not a fighter". TW4 better contain more realism, more optional micromanagement, and defenitely a smarter AI in all levels and aspects. If it will it could be the gratest game of all times.
TW is IMO the best game there is - period! Civilization once was that but have moved in a unrealistic direction. The Caesar/ Hellas/ Pharaoh series was also a very good game but just focused on building. EUII had the best diplo-system ever built into a game - period. A combo of theese games would eventually prove to be the ultimate game. The devs has to keep in mind though that all feat must be optional for the player. A "dominaor" just want to fight, a "builder" want to focus on that and micromanage his empire etc etc.