Yeah they have great cavalry. But it's a damn shame that have crap land and I found it hard to do seige with themParthia. There are few things more satisfying than smashing a vast enemy army with nothing more than a few primitive horse archers.
The seleucids kick as well, but for some reason every single nation I border ALWAYS declares war on me and i have to fight a seven front war...
Eh, the Scythians/Sarmatians prolly have better horse archers (never really played either) but their geographical position is arguably worse than the Parthians. Plus the Parthians can eventually get Cataphracts and Elephants (though admittedly, by the time you have them, you shouldn't need them).