I think the Japanese concept of "Shibumi" describes well the reason why STW stands out to many over MTW and (I assume, as I haven't played it) RTW.

Now that I've played a bit of MTW-VI MP, one thing that I like about it over STW is that Light Cavalry (which would equate to Yari Cavalry in STW) are not as "sticky" in MTW-VI, allowing for more effective "hit and run" tactics--a primary function of this type of cav, imo. Yari Cav have the speed over other cavalry types in STW, but once they engage, it's very difficult to disengage them. You can do it, but you have to do it very quickly after they impact, and their kills for that charge aren't usually very high. Alans, Steppe, and even Mounted Sergeants and Hobilars in MTW-VI can work wonders using "hit and run" tactics, particularly in MP, if used carefully and timed correctly.

Hands down though, my favorite of the series is STW and I don't see that changing any time soon.