Well I feel like a noob...
Just approaching 600 hrs on Warband.
Next is probably 200-ish hours on ETW, although Crusader Kings II might have surpassed that (I play it off of steam a lot in it's own directory so I'm not really sure...)
Both RTW and Medieval II probably have well over a hundred hours, but I can't say for sure.
Actually now that I think about it, the only PC game I ever played when I was younger was Cossacks, so it could be ~200 hrs as well... not really sure.