I have several breeds, but I've been selling a lot of chicks from my FBCM and American Game cross. They do well in heat. Games are great layers and the hens are usually very mellow. With this mix they will lay a brown egg although not as dark as a Marans. While the Game eggs are usually just large my FBCMs lay a huge egg. Thats the first thing people tell me when I ship them. They've never seen them that big. The other reason I do this mix is for meat birds. I just can't eat a Marans roo, so I mix them. I hear Marans are very tasty, but these will do.
The thing about Leghorns is although maybe not aggressive (mine were) they tend to be flighty, but if you are looking for good layers, they lay like crazy.
Another breed you might like are Dominics. Although similar to Barred Rocks, I like them much better. Not as aggressive and lay like there's no tomorrow. I used to have one that would occassionally lay twice a day, only missed laying maybe once a month and if you need one they are also great broodies. Just don't touch their babies. LOL