For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
Back in the day I installed Rome Total Realism and oh man, it's so good, way more provinces including some tiny city states and you can do way more tactics because armies don't die off and retreat faster than it takes for a unit to walk 100 yards. The original creators didn't want to force you to use fast forward. You just use fast forward for RTR and it's perfect
Oooh i never got into mods for RTW. I might give them a try.