Mages of Order in Heroes IV can actually prove to be good summoners, creating 44+4 per level HP of creatures with Create Illusion and 88+8 per level with Phantom Image, or 152 HP at 8th level (the minimum when level 4 spells become available). For comparison, Summon Griffins, level 4 Nature spell, creates 0.2*level Griffins. As Griffins have 95 HP, they effectively summon 19 HP per level, or the same 152 HP. Each level after that Nature summons 11 HP per level more than Order, but... there are also Genies that summon 18 HP per Genie, summoning being arguably their best utilization. Not bad, aye? So, not only is Order the direct counter to Nature with Banish, but it is also better at summoning than Nature, also having many more interesting mechanics to choose from. But that's not the point.
The downside of Nature summons is that you can only summon the creatures that you got in your Mage Guild, while the downside of Creat Illusion is that you need at least one creature of the respective type to be present on the battlefield for at least the first round(s), but you can summon litereally everything except for Black Dragons.
Not only do you usually not have a choice with Nature (especially at higher levels), but you can also just compare the few things you got to choose from when you do and pick the best. With Order it is a lot harder. Either you have to rely on your enemy to bring the prototype, which doesn't happen always, or you have to carry one with yourself. So, the question is:
How to find the best creature to clone in any specific situation?
The rules of thumb and vague guidelines that I follow for now are to aim for a creature with the most Defence, because this means that my summoned creatures would last longer against normal attacks. I also typically aim for high-leveled creatures, because they are typically stronger, plus normally have higher Defence value.
I also typically aim for melee fighters because they are disposable anyway and typically deal more damage than shooters and, what's more important, bind the enemy by engaging them in melee fight.