I do think that 'worst' describes the Thracians pretty well. As previously mentioned, their starting position is similar to the Seleucids, but unlike the Seleucids, there isn't any reward for putting up with early-game difficulty-- they don't get cool late-game units like the Seleucids do. In fact, the power of units on their troop roster decreases with time. Their late-game units simply cannot compete with theier late-game counterparts from other factions.