Both GHRP6 & 2 are very popular for the hunger sides and also the GH releasing potential. I would state that they both work but where Ipamorelin outshines them is there is for 1.) no saturation ceiling with Ipamorelin and 2.) there is no rise in prolactin or cortisol.
Which GHRP 2&6 both do so they are a little sloppy when compared to Ipamorelin.
As for MK677 according to this study they measured the GH secretion along with cortisol levels and the data showed that MK677 did not affect cortisol increase due to elevated IGF-1 levels. This means MK677 like Ipamorelin is cleaner than GHRP2&6.
http://www.ncbi.nlm.nih.gov/pubmed/8768828