Did you ask this question a couple of days ago? 'I've never been as at peace with myself as I have in the last week' = just comparing to before 'I've never been so at peace with myself as I have in the last week' = wow! a new high level! (it's just nuance)

Have been is more normal. 'Had been' sets you up for a change of some sort. If you followed this by something like 'that was until...', then 'had been' would be OK. So and as are both OK. (so/so much/as much) all sound fine. Bare 'as' doesn't, due to the doubled a sound of as at.

@Flavio - you incorrectly changed 'at peace' to 'in peace'.

,,

Jordana, I think the best construction here is "I have never been so much in peace with myself as I did the last week" Notice that this phrase has two parts, the first one you have to use the present perfect "I have never been so much in peace with myself" (there's no indication of time) and the second part: "as I did the last week" (as you indicated the period when the action has occurred, you have to use the simple past).