The best companion
 

Which companion is better, Agvares or Gaudi?

I vote Agvares. He's always my favorite (2nd place takes Elenhel). Even though his special abilities do not affect most of the units, he's got very versatile slots (another weapon and artifact slot, armor/dress, belt/gloves), that suit pretty much any hero.

Gaudi doesn't have an artifact slot, which is bad for a paladin or a warrior hero, who has only 1 (and some artifacts are well worth using them to boost the things you lack - mana or rage, for example). His ability to improve the crit against undead is nice (practically doubling it), but against vampires, who cannot be critted (because of their special ability), you may find yourself in a battle you cannot win unless you drop your basic crit rate dramatically :)

I meant Agvares because of his ability to add special abilities to units he controls, which extends your tactical options although you can't control them but you still can cast spells on them, like teleport or berserk or target.

Btw, wouldn't Gaudi be much better against Baal?

He is :) But I would take him prior to going back to Greenwort, because Agvares' flexible slots will be very useful against K'Tahu and Zilgadis.

Why is that?

With Agvares, you can put 2 weapons (Singing Dagger and 20% Crit, for example) as a Paladin or a Mage, which will make killing them way faster. With him, you can put on 2 slippery cuirasses, or a slippery cuirass + leather dress. The other artifact slot can be useful because you can slap in +30 mana or an ancient amulet for +4 Int/15 mana, which is great for a Warrior.

As for Gaudi, +50% crit against demons will help greatly in the final battle.

I may be wrong in someone's eyes, but what KBAP taught me well is that flexibility is the key to success.

Too bad you can't have your own castle to leave all your companions in and paying them upkeep each day, if you have more than one.

Well, that would make the game look a lot like HOMM :)

Nothing wrong with that, some aspects of HOMM5 are very good, like seeing your heroes being actively involved in the battle, some spell effects, fort sieges etc.