Aven Cloudchaser wants to destroy an enchantment when it comes into play. If there are many enchantments in play, you pick one. If there is only one enchantment in play, you must pick that one (even if it is yours). If there aren't any enchantments in play, the ability doesn't happen. In all cases, the creature remains in play.
Orzhov Basilica's ability reads, "When Orzhov Basilica comes into play, return a land you control to its owner's hand." If you control many lands, you must return any one of them to your hand. If it is the only land you control, you must return Orzhov Basilica. It is the ability that is returning it to your hand, not anything in the rules.
Unless an ability says "may" in it, you have to do it - even if it is detrimental to you. If you can't do the ability, just think that it doesn't happen.