Yew longs aren't the best in terms of cost, but they are definitely the easiest to acquire. Green and blue dragonhide bodies also work well, but it's much harder to get them in high amounts. As always, it's impossible to buy more than 100 at a time through the GE, so forums are your friend. You'll probably get several offers to buy thousands of yew longs within five minutes if you post on the RS forums.
Oh? The only major exception I can think of is sapphire bracelets, where you lose 0 gp (depending on prices you can lose 1 or gain 1 gp per cast, actually), but you have to make those. Please give me examples of being wrong, I'd love to see them.
I'd prove him wrong myself, but I hate running out of the heroes guild when I don't even have a half load mining. Trust me, though, he's wrong. Any item where you make a measurable profit through alching is either extremely hard to get or only remains so for a short time. Once alchers figure out about it, the demand goes way up. When people compete over the supply, the price rises to a point where the supply evens out the demand again, meaning it is no longer profitable. Simple economics, but apparently far over SCREAM's head.