They all have their advantages:
-RA is more reliable the Slipher, but less reliable then Obelisk.
RA's Ability allows it to turn the tide of battle via Special summon.
-Slipher has the most potential and is my personal favorite. Slipher's ability is beautiful...
-Obelisk is the most reliable, but also the most pointless. 1st of all his ability sucks...and here's why:
1) When you summon Obelisk you probably don't have 2 monsters to spare
2) By the time you get two more monsters on the field you should of already won
3) His ability's cost betrays the purpose of it's effect - If you have obelisk and X amount of monsters on the field chances are your opponent has little to none on their end making the point of his effect null.
So he's basically a powerful monster with immunity to target spells/effect/traps...
Usually when you summon a God card you already have the upper hand. This means usually the opponent doesn't have many monsters on their side of the field. This is what makes Slipher shine... Even if he has 0 Atk and 0 Def he can still be a bad *** and that is why I love him. Just be careful - DONT attack face downs if Slipher has low atk or you will null Sliphers (2000def or less = death for enemy) effect.
Oh and Ra is awesome in the grave yard... Just keep special summoning him for raigeki fun :D