I'm big on hiring mercenaries, but they can be expensive so you have to be careful about when and where you hire them and how to get your moneys worth out of them.
Aside from siege engines, I always hire Vikings whenever they are available, I will not pass them up. Compared to other merc units, especially calvary, they give a lot of bang for the buck. For an effective infantry unit, they are low maintenance and low cost. I usually try to build for producing high value spears, pikes, and halbediers, so I'm usually a little short of swordsmen. The Viking mercs fill the gap well. There are lots of merc units that are useful for a lot of different situations, but these are the only ones I never pass up without fail.
If Alans and Steppe Heavy Calvary are available, I will almost never pass them up #- again the most bang for the buck. Druhzinas, Faris, and Byzantines are great calvary but very expensive - they will drain your treasury quickly.
Which mercenaries, if any, will you never pass up at the Inn?