Numidian cavallery would be the most usefull one, but I always get them from regional mics.
The mercenary I am using the most are the mistophoroi pezhetairoi. You can never have enough phalangitai, when facing factions with heavy cavallery
But my favourite are the caledonian skirmishers, the Balroae. They are incredibly strong, esspecially in woods and they are cheap and spawn incredibly fast. Man I beat a full stack of casse, just with mercenaries and the Balroae did most of the damage.