It's a complex question. There isn't really a universally "best" monster here. While you could argue that a ballista is the best ranged attacker, it significantly slows down the army on the march. It all depends on the specific situation. In large numbers, any of these monsters will pose a threat. I think I'll vote for golems because of their resilience (usually in defense). Tigers and nomads are quite similar in their stats and are designed for attack, but despite their initial strike, they aren't very durable in a fight.
Beholders are decent, but they're only useful as mages. Fighting against them is unpleasant, and you can't build a strong attack force out of them.