As far as the reviewer is concerned, ME1 is not the best in the series. Saying it is is mere opinion, not universal for everyone.
It would be totally fair to give the sequels higher scores if the author deemed them worthy, since reviews are ultimately opinion pieces.
A 6/10 is not a low score. Anything below a 5 is.
I'm sure that you could compose a much better one, given your exemplar writing skills.
Yeah, cause the lead editor for an exclusively PlayStation-based website is totally a "next box fanboy". Get real.