Can dark hole kill God Cards?
For a short answer, yes. On their own after they are summoned they can be affected by card effects (obelisk excluded if the effects target). For an o.g. example though, dark hole will destroy any God card. But even with this card out, your gods are still susceptible to untargeted stat changes.
Can Egyptian god cards be destroyed by card effects?
The opponent cannot equip cards to the "God" Cards. They cannot be returned to their controller's hand. They cannot be destroyed by card effects of non-"God" Cards. However, it seems they can be removed from play (as Yugi removed from play "The Winged Dragon of Ra" with "Ragnarok").
Can obelisk be destroyed by dark hole?
Judging by the real-life world's rules, then yes. Obelisk's real-life effect only protects the card from being “targeted” by card effects. Dark Hole destroys cards without targeting, so Obelisk can be destroyed by Dark Hole.
What is the weakest Egyptian God Card?
Obelisk has the weakest potential. Seeing as he requires a minimum of 5 monsters to get his infinite attack for one turn, I don't see how he is better than Slifer, which can destroy almost any monster summoned, which is what real gaming is about, destroying monsters as quick as possible.