Poetic Atmosphere TPU
TPU cases are extremely popular considering they can take tons of abuse, are thin, and also happen to usually be extremely cheap. Usually $17, the Poetic Atmosphere for the new HTC One M8 is only $2.95 on Amazon until the end of April, and it comes in a few different colors.
What's special about the Poetic Atmosphere is it's made from a durable polycarbonate hard shell for protection, but the corners are TPU (a soft plastic basically) for added durability, shock absorbent, and it's also easy to hold without sliding out of your hand. It won't ship til the end of the month, but get it now for under $3 while it's still available.
$2.95 from Amazon