Depending on their skill set and class, almost all mercenaries are suitable. The Act 3 mercenaries are at a significant disadvantage due to their caster role and the limited number of items available to them, causing them to fall behind.
If you could give them amulets, rings, shoes, belts, and gloves, they would certainly be more useful. But for a while, an Act 3 Frost mercenary is actually quite useful for CCing enemies.
Or a Fire mercenary with maximum +skills enchanted at a high level can do quite well in Normal mode against small players with fire damage.
The Act 1 mercenary, on the other hand, is very well suited when it comes to support in the form of, for example, Insight Aura, Purify, or Thorn Aura.
Especially when a sorceress needs Insight and not Infinity, the Act 1 mercenary is the best choice, simply because she hardly ever dies due to teleporting and staying away from monsters.
The Act 2 mercenary is actually the most boring mercenary, but very useful due to his auras… but I don’t like him.
My favorite is always the Act 5 Barbarian Mercenary, followed by the Act 1 Mercenary. Only when I’m forced by the mechanics to take an Act 2 Mercenary, e.g., for a Strafezone for damage, do I take him… or because of RW like Infinity… But the guy is a sleeping pill…
An Act 5 mercenary with strong equipment is a machine and also fun…
I always prefer RW Plague, for example, or if he’s supposed to be a wall himself, then RW Grief, Last Wish COH, and helmet, depending on what you need, and let him go…
2x RW Crescent Moon was also murderously fun…
Whether I want to lug around 2 mercenaries and equip them both hmm… Depending on the class and skill level, that would certainly be helpful… But then, of course, it quickly becomes very dependent and probably favors certain classes even more than it already does.