*Mid-game Spoilers*

Cloud goes through the whole game suffering from the mind control of Sephiroth (as far back as when he passed out at the second reactor for a short while). He even almost cleaves Aerith's skull in half and hands the Black Materia to Sephiroth. Through some miracle, the heroes manage to get it back before Sephiroth obtains it. Cloud decides to leave it with a party member since he doesn't trust himself and fears Sephiroth might take over him again. So he gives it to Barret/Nanaki.

Barret is shown to be the leader of a resistance team to Shinra. Barret as the leader more than likely does a fair share of thinking about how to get to the places and the measures they need to take to complete the mission and escape.

Nanaki is shown to be a clearly intelligent creature several times throughout the game. Upon first meeting him, able to form a plan with Aerith to avoid Hojo doing something worse to the both of them and leaving them helpless.

WHY THE *)^% DO THEY GIVE THE BLACK MATERIA RIGHT BACK TO CLOUD (after he's brainwashed by Sephiroth mind you) NOT FIVE MINUTES AFTER THEY GOT IT!?!

Actually, Barret later said that he wasn't cut out to be leader, which is why he willingly stepped back and allowed Cloud to assume that role very early on. Even when Cloud disappears after Meteor is summoned, he still insists on someone else taking the reigns (namely Cid), although he still provides advice. He also holds Tifa in high regard as well, treating her almost like a daughter.

Red XIII however, despite his obvious intelligence, is very innocent when it comes to human society. He seems to be very trusting and takes things at face value.

Hence it's quite easy for Sephiroth to use his Jenova powers and assume Tifa's appearance to trick both of them into handing the materia over. They themselves say they were uncomfortable holding it and were quite happy to get rid of it.

Yeah, but giving it back to Cloud after him saying he doesn't trust himself? I could understand giving to someone else, but Cloud has been possessed so many times before this that its just silly to do so.