I prefer bribing rebel armies, as they are quite effective. Or, when a faction is dead, just bribe all of their armies as they are quite cheap to hire.

Also, it depends on the general. If they have a special statistic that makes them easy to bribe, then do it. If an army costs more than 10,000 florins, then, it isn't really worth it. But, look for low-loyalty generals, as they are pretty cheap to hire.