Studying laws is a very variable thing. I think this mechanic was added by the developers primarily to get rid of a single guide and make each game flexible. 

Therefore, let's consider not an exact choice, but the rules that influence which laws are worth studying. 

Limit of 100 Law Seals

You cannot spend more than 100 seals, and they won't be enough to study all laws—you need to choose what will be studied in this game. So immediately estimate: is it important in this game, will you be able to get 100 seals by the end of the game? If the map is small and there is only one hero, then most likely not, and you can ignore this limitation. 

But the Temple has many abilities aimed at increasing the number of law points, so a very rapid growth is also possible. 

Economy

In my games, I try not to use law points for one-time acquisition of resources, gems, or gold. They give relatively little, but a valuable and finite resource will be spent. Economic laws are worth using only if the boost for buildings is really important, or if we have a severe resource limitation on the map. But as a rule, it's easier to hire a hero with resources at the tavern or actively clear the map. 

Creatures

Since laws give a strong boost to the army, both in quantity and quality, upgrading all creatures is a must. 

Mandatory

Heroes of Legends - morale is never superfluous, and high values very often greatly reduce losses or even turn a clearly lost battle into a victory. 

Quick Reflexes - increases initiative, which is indispensable in battle, especially in conjunction with artifacts that provide a similar effect. 

Training. Melee - upgrade by 2 levels, +10% to attack and 10% damage reduction are not found lying on the road. 

Training. Ranged - we have three types of shooting creatures in the army, and angels are 7th tier, so it's also a mandatory skill, upgrade by 2 levels.

Training. Attack - upgrade by 2 levels, even for a mage hero, the army is very important. 

Training. Defense - upgrade by 2 levels, even for a mage hero, the army is very important. 

Inspiration - a decent law on its own, but mandatory if you are leveling leadership. 

Vengeful Strike - mainly needed in final battles, but very useful there. Learn towards the end. 

Hero's Blessing - Attack and defense of friendly creatures increase by 25% of the hero's spell power and knowledge respectively. Especially useful if you are learning battle magic. 

Situational

Inquisition March - not worth upgrading immediately, but if we see that we are facing a Rift hero or a summoning specialist, then it's worth upgrading. 

Striving for Perfection - only if we took a hero with buffs and plan to develop through magic. 

Training. Spell Power - only needed for a mage or if you are studying the "Battle Magic" skill—there is a skill that transfers 15% of spell power and knowledge to creature attack and defense. It also synergizes with the Hero's Blessing law.

Training. Knowledge - only needed for a mage if knowledge is low, but usually not required, or if you are studying the "Battle Magic" skill—there is a skill that transfers 15% of spell power and knowledge to creature attack and defense. It also synergizes with the Hero's Blessing law.

Sacred Lands - situational, only if you are playing against a mage with a debuff specialization. 

Doubtful

Training. Scouting - gives +1, +2 to scouting. A useless skill, both in abilities and laws. 

Faith in Magic - since astrology points allow you to upgrade only travel spells, a sufficient number of points accumulates quickly enough to get 1-2 necessary ones, and over time upgrade them. Moreover, if progress is very slow, you can get additional growth through castle buildings. 

Power of the Sun - useful for a mage hero, but the effect is small, and if you already have a magic school in abilities, the level will be max anyway. 

Skilled Architects - discussed in detail in the article "Is it worth studying the Skilled Architects law in the Temple faction"

Power of Faith - Allows heroes to learn spells of any level, but a mage will already be able to do this through magic school abilities, and others don't really need it; besides, this is the 5th level of laws, which will be towards the end of the game and won't be useful there.