firstly the Illyrian hippeis, admitavely I don't conquer illyria for that sole purpose but I buy them as soon as they are availible as mercs^^.

then there are (most) balkan troops. its mainly the elites foud there which often "force" me to roleplay one of my Generals is thirsting for glory, takes his "own" troops adds some mercs and runs amok in tracia, as conquering the Getai is actually not part of my regular KH-agenda. tracian prodromoi may not be really superior to normal ones but having them is sort of a prestige thing: hey look everybody else only has nomal prodromoi I've got "special" prodromoi.

last but not least I'd like to mention unique greek colony troops I often cross vast distances to get my hands on.