STW had by far the best game-play. I did, however, like the addition of multiple-direction routing added in MTW and the map size increase of MI.
Graphics? They have improved in each version, and in some ways it is difficult to look at the old STW maps.
The RTW units look fantastic, but there is something a tad "small" about them when zoomed out that makes them difficult to click on. The MTW and RTW trees annoy the hell out of me, forest fighting has only gotten worse from the STW days where I used to love getting into the trees.
I am also a HUGE supporter of HTW. I think it is near the tops in SP and the limited MP I've done with it has been a lot of fun.
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In RTW SP, with the transition from campaign-map to battle-map and the ability to move within a province are great additions. However, for me, those improvements are far outweighed by all the massive changes that have hurt battles (both in SP and MP) and the battles were reason I play the game. Also, the fighting in cities was a great idea, however I've found the execution of it to be, as of now, too buggy -- especially with pathfinding-- to consider it a plus.
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