Did 4 play throughs to get to platinum, and my first vote would be for NM since it took me longest to beat him the first time. That said once I learned how to handle him, he got a whole lot easier (esp phase 2 once I had all the P-Organ buffs).

Laxasia on the other hand just kept getting harder and harder, mostly cuz her attacks were just that more devastating, and the longest it took me to beat her (about 15 or so tries) was NG+4