Hmmm...I think I'll rank them from least to most cruel, all in my own opinion, of course -
Lust - I'd rank Lust as the least cruel because I think she changes the most throughout the course of the show and ends up the most human. She makes a huge sacrifice to help Ed just before she dies, and even though she had her own personal motivation for doing so, I think she had also developed a sense of morals and sympathized with Ed and Al, as well as realized that she couldn't let the stone fall into the hands of someone like Dante. Sure, she caused a lot of mayhem and destruction, but I think she redeems herself, or at least tries the most to redeem herself.
Greed - At first glance, he seems like he could be a top contender, but when you think about it, he really is one of the least cruel, perhaps less cruel than Lust, depending on how you look at it. Yes, he's greedy and wants everything for himself, but he also frees a lot of people from Dante's tyranny. Sure, they only shift from being under Dante's control to being under Greed's, but all of the Chimera are extremely thankful to him for freeing them, and he certainly treats them a lot better than Dante did, and he does seem to care about them, since he regards them as his. Then he kidnapped Al, and made some threats, but hey, he let him go, and even refused to fight Izumi, didn't he? And then there's his dying act - to tell Ed how to defeat the homonculi. He had ulterior motives of revenge, but at the same time, he was helping Ed, so I would say he was definitely greedy, but not to a cruel extent.
Wrath - Ok, I'll admit...I love Wrath. He's my favorite character... soooooo, I might be a bit biased. But I think his cruelty can be somewhat excused in that he really is just a highly confused child, often to the point of desperation. I think Izumi's handing him over to the gate was far more cruel than anything he does throughout the series. He's just a small child who gets caught up in a lot of things he doesn't really understand, and longs for a mother. Even though he hates Izumi, his utter devotion to Sloth is kind of sweet, albeit a bit misguided. And if we're counting the movie, then he definitely gets a lot of redemption points which could bump him a bit further up the scale, so to speak.
Gluttony - He was a hard one to place, since he is probably the least characterized of all the sins. The only thing that isn't cruel about him is his inexplicable love for Lust(Well, maybe it's just that she's the embodiment of Lust XP). Otherwise, he doesn't really seem to have any sense of morals, and just wants to eat everybody he sees, sometimes getting rather scary in the process. I guess you could make the argument that since he's pretty mindless, he's not really being cruel, but still...eating innocent people...not nice! And he doesn't really have any redemption, either, so yeah, I'm putting him in the middle.
Sloth - I'm surprised she didn't get more votes. Wasn't anybody else rather horrified at her motivation for trying to kill Ed and Al? She knew they were her sons, had memories of being a loving, caring mother, and rejected them out of fear of not being able to live up to them, going so far as to try and kill her sons in an attempt to liberate herself from them. You could almost call her greed. And she manipulated the fact that she was, or at least once was Ed and Al's mother, trying to use it against them to protect herself, or take them off their guard. Really, that is incredibly cruel. I'm tempted to make her the 2nd to most cruel, but she does have a tiny bit of redemption in her last words, during which she seems to accept that she is/was Ed and Al's mother, and tries to comfort and encourage them, though it seemed to really just freak them out a bit more...whoops!
Pride - 2nd to most cruel in my books, Pride took a lead role in corrupting the government and military, and brought about a lot of war and destruction in doing so. Of course, these were all higher orders from Dante, but he still had little regard for anybody's life, and had entire countries whiped off the map without so much as a second thought. He also has a bit of a pleasant facade at first, which makes his cold hearted cruelty all the more noticeable later on, especially when he is being nice and fatherly to his (adopted)son one moment, and strangling him the next. No redemption for him, either. He's just a ruthless bastard.
Envy - Really, Envy is quite undeniably the most cruel of the 7 sins, hands down. He's not just a ruthless bastard, like pride, but a purely evil
. And he enjoys it! Either while he's kidnapping Al or after he's kidnapped Al, he reveals that he isn't even in it to become human. He knows that Dante is just going to use the stone for herself, and doesn't care - he just wants to kill humans! For the pure fun of it, presumably. And killing Hughes while in the form of his wife, man, that's just sick. It's completely pointless cruelty, he's just being cruel for the hell of it. He pulls the same trick during his big fight with Ed, too, transforming into his dead friends, and even his mother, in an attempt to break him mentally. We can hardly blame him for hating Hoenheim, but hey, Ed hates Hoenheim too, and he's not a cruel character at all. Envy is just an evil, obsessive, insane bastard, and all of the other homonculi pale in comparison in terms of cruelty.
Soooo...yeah...can you tell I've been bored today?