Out of the five, #s 1, 3, and 4 were done the best.
Clouds #1 had a great color to them, and the shading was done very well. However, I feel the light source was a bit, I don't know, unnatural? But that shouldn't change anything ^^
Clouds #2 honestly does not look much like clouds, but more like mossy-rocks in my opinion. I feel that the "puffs" and "curves" of the clouds are not light enough, making them seem hard and un'cloudlike'.
Clouds #3 is personally my favorite of the 5 ^^ They are very realistic, light, fluffy, and the blue balanced out with the white perfectly. Good work
Clouds #4 is very dark and mysterious, perfect for a night-time scene. Maybe it's just how the picture is angled, but I think the cloud shape is a bit too dramatic in slope. Then again, clouds can take many shapes and forms, and maybe you were going for this. Good work
Clouds #5, something I can imagine being on the surface of a foreign planet, like Venus or Mars. The shading and lighting here is perfect, and the shape of the clouds are nice too. I just feel the clouds may be a bit too 'plump' for their width. Then again, clouds take many shapes ^^