Firstly, you have to understand that every spell on Populous can be effective, it only depends on the situation. Depends on so many things, such as how many tribes you are facing, how many population you have, how many troops the enemy have, how many wildies are one the map...also depends on the shape of the terrain, the ammount of puddles, vehicles, etc ,etc. 

 

Still, some spells are better than the others. For instance, if a player choose between having a free shot of blast and a free shot of volcano, 99,9% of the times a player would prefer the volcano, obviously.

Many players agree that the most effective spells are, though, the blast, magical shield, land bridge, and, ocasionally, the convert and ghost army. This opinion is personal, but, generally, is correct.

 

 

Blast - Extremly cost-efective spell. Really cheap and destructive against units. This is the most used spell in Populous and mastering the blast is half the way to suceed in this game.

 

Convert - If there are wildies on the map, it is extremly cost efficient, since it's the cheapest spell, and free braves is always good. If there are no wildies, it makes no sense to charge it.

 

Swarm - Somewhat cost efficient spell in most stages of the game, specially in later stages, where it will charge fast, and will help alot.

 

Invisibility - In levels with preachers or warriors, this spell can be very cost efficient, since it creates deadly atacks. 

 

Magical shield - extremly cost-effective spell. It's cheap and grants deadly status to troops (the imunity to spells and fireballs). Because of the strenght of this spell, it is usually off in online levels.

 

Landbridge - Very cost efficient. An earthquake costs way more and could damage less than two buildings, when a cheap landbridge would make room for 2, 3, 4 or 5 new huts. The only good expanding and recovering spell, so yes, it is really effective for a relative cheap price.

 

Lightning - Lightning is usually not really cost efficient. Just look at it: it surelly kills any unit, but what if you miss it? most of the times, you can do with 1 or 2 blasts what you did with 1 lightning. The really pro of this spell is to kill shamans in online gaming, where it can actually decide games.

 

Hypnotise - A medium cost effective spell. It affects only grouped units, so in online playing, it's not always easy to use, since the enemy will split the troops. Still, this spell can be strong for relative cheap mana.

 

Tornado - Also medium cost effective spell. In some levels where the settlement's buildings are split, this spell works well. Also works good against defensive firewarrior towers.

 

Swamp - This spell is deadly. In singleplayer, it is extremly cost effective. In online Populous, not so much - the players are smart enough to dodge it and create cheaper landbridges to avoid it.

 

Flatten - Most of the times has a low effectiveness ratio. The exception is against training buildings near water, or to use it to gain advantage of hills.

 

Earthquake - Cost effective in many levels where the buildings are together - "Face off", "Pressure Point" - not "Dead Sea" though.

 

Erode - Probably the less cost effective spell in Populous. It costs alot of mana and can easily be countered with a cheaper landbridge. Most of the times, you can do with a flatten what you do with a erode, but this spell is still good against settlements built on hills, or when combo'ed with earthquakes.

 

Firestorm - This spell is somewhat cost efficient in later stages of the game, where you have good ammount of mana income. It is good against units and buildings, and will free your shaman's way to the heart of the enemy's base.

 

Angel of Death - Low cost effective spell, unless you are playing singleplayer, where it is decently good. Online, it can easily be avoided by 2 to 4 ghost armies, which are extremly cheap.

 

Volcano - Like the firestorm, it is somewhat cost efficient in later stages of the game, when you have good ammounts of mana. If you think you can get inside the enemy's base, it could be a good idea to charge it.

 

Ghost army - very cost efficient spell, specially for when atacking, to make the enemy busy. Will make your shaman survive many times.

 

Armageddon - extremly low cost efficient spell, since it takes ages to charge, and it might just lose you the game. Just too risky for a high price.

 

Bloodlust - We can't deny this can be the best spell of the game, but it takes ages to charge. You could charge devastating spells by the time you got 1 charge of bloodlust.

 

Teleport - Like any guest spell, it is not cost efficient, as it takes many time to charge. Still, if you can not get past the enemy's defense, but you have control of the game, you can try it.