The fighter is a disciplined warrior. They are versed in the use of all weapons and armour. They are valuable tacticians and incredibly stalwart companions. With the addition of the feats, they make for incredibly resourceful characters.

Fighters need a high Strength and Constitution. Dexterity does little for them, since they will be wearing plate mail anyway.

The executioner is a savage fighter, a merciless killer of his foes. Able to go into a mindless rage, they are easily the equal of the disciplined fighter. A high Strength, Dexterity and Constitution help an executioner immensely. They are not proficient with heavy armour, so the Dexterity bonus will keep them alive for a bit longer.

The paladin is a holy warrior trained in the use of most weapons and armour. They trade a lot of their combat feats for special powers, such as turn undead, spells, protection from evil and the ability to smite undead.

Paladins don't require any feats or skills and should have high Strength, Constitution, Wisdom and Charisma

The ranger is a protector of the wild. They get many special abilities that the fighter doesn't, including the use of some spells later in their careers. As long as they wear light or no armour, they get the Ambidexterity and Two-Weapon Fighting feats for free.

A ranger needs a high Strength and Dexterity, as well as a medium Wisdom. Wilderness Lore is their only required skill.

Clerics are primary healers and givers of protective spells, but they also have great combat potential, both at melee and offensive spells. Give a cleric some Concentration and spellcasting feats.

Clerics need high Wisdom and Charisma, and a mediocre Strength.

Of the two magic-user classes (wizard and sorcerer), wizards are the more flexible of the two and also tend to be a bit more accomplished, both because they tend to have more skill points (from high Int) and because they gain extra feats. The fact that wizards can learn a wide variety of spells can be key, because for difficult battles you can pick and choose your spell selection to meet specific tactical goals.

They specialize exclusively in elemental magic, and all of their spells fall into one of three domains: cold, lightning and fire. Focusing in spells that deal heavy elemental damage and starting with the biggest mana pool. In melee battle they are very weak, but can still function in hand-to-hand combat with the right gear and skills