Insane Strength Potion

From a level 80 ret pally perspective....

This is a good potion when you first start raiding, but becomes weaker than the Potion of Speed as you gear up.

A fresh level 80 pally will have around 3000 buffed attack power. With talents and kings this potion is worth 300 AP, or about a 10% increase in dps. This same Pally would see about 8% increased dps from a Potion of Speed (3% more melee damage + 1% seal of blood from melee hits + 4% from lower global cooldown tightening the rotation).

A pally in half naxx gear will have around 5000 buffed attack power. The extra 300 AP from this potion becomes about a 6% dps boost. The haste potion on the other hand scales with attack power and retains the 8% dps boost, making it superior to the Insane Strength Potion.

A pally in full naxx gear with judgement cooldown reduced to 7 seconds and AP approaching 6k will see even a greater benefit from the haste potion, and less from the strength potion.

This is currently the best potion for melee DPS in WoW

Haste Potions are not able to be used through levels 50-59. They are quite nice once available, however.