Which of three Egyptian God cards is the strongest?

Well, for the legal play ones, I'd say by far would be Slifer the Sky Dragon as it retains most of it's power from the anime, gaining 1000 ATK&DEF for every card in players hand and the -2000 to opponents Normal/Special Summoned monsters ability. Granted the second ability only deals with ATK power and the monsters have to be in ATK mode.

Obelisk has Raw ATK strength and can't be targeted by opponent card effects(cards that say Target Monster), but is still susceptible to battle loss and non-targeting effects.

legal play version of Ra is kinda seen as the weakest or hardest to use on the account it doesn't gain the ATK & DEF of the tributed monsters, but for it to have ATK & DEF you'd pay until 100 LP remain and Ra gets that much ATK & DEF...but it's Original ATK & DEF is 0, so Megamorph wouldn't work with it. There is also a "Pay 1000 LP to target and destroy 1 monster" effect...you can probably see why people would have a hard time playing this card.

It depends on which strategy you use in dueling.

If you use a deck that relies on keeping a good amount of cards in your hand, Slifer the Sky Dragon would be your choice as it's attack and defense are dependant on the number of cards in your hand.

If you are playing an 8000 life point duel and have many cards to regain lifepoints, the Winged Dragon of Ra would be your choice.

Last but not least, if you like generally high attack power and a backup effect that clears your opponent's field to prevent your opponent's backup plans, Obelisk The Tormentor is your choice.

All three have weaknesses, for example, if you have few cards in your hand, Slifer is weak and can be killed by an attack by a monster.

If your opponent manages to deal you even 100 damage directly after you summon the Winged Dragon of Ra, you will lose automatically.

Finally, Obelisk The Tormentor can be beaten by attack power if you equip a monster with enough equip cards that increase attack and his effect does not work without two monsters to tribute.

In the end, it is up to one's strategy to determine which of the Egyption God Cards is strongest.

Just get all of them problem solved