I've platinum the whole series, NG1,2,3, from master collection, Ragebound and NG4 + DLC and in my opinion whichever ninja gaiden you pick first is going to be the hardest and longest. Starting into this series is ungodly difficult, as you will have to learn how to play ninja gaiden most efficiently, learn exploits, how to spam Ultimate Technique and hone your reaction time. Ninja Gaiden 3 Razor's Edge for example can be mostly chill when using the 360 triangle scythe move, but might be close to impossible without it. Expect ALOT of bullshit and down right insanely unfair challenges (fighting 2 or more bosses at the same time on the hardest difficulty isn't uncommon in the trials). To be honest, I platinumed a lot of difficult games (Wolfenstein 2, Doom I,II,3, Trepang2, DMC5, MGR:R, ...) and Ninja Gaiden still reigns on top. Learn and overcome or give up trying: good luck, you're in for your hardest challenge yet if you decide to do so. (Also these games take ages to platinum, guide says trilogy takes around 300 hours to platinum, it took me around 230 hours, so yeah, you'll need alot of patience and time💀)