I like #153 and#154 best. I think becaus they have a softer feel to them. The colour is really good.
Can you try rotating it so the M is standing more upright?
You could also try with more of a rainbow effect on the M - either the entire rainbow or just the green-blue-indigo-violet section of the rainbow.
Just so you know, there are a lot of entries using the M - that's not necessarily because that's what I wanted - it's just that my favourite entries just happened to have an M and then there was a snowballing effect. So if you want to try any non-initial related entries that would be good.