I'm very interested in Ninja Gaiden reboot as I already played Ninja Ryukenden games but... the problem is... that Itagaki forced the team to delete the source code of the games that he worked and trapped in OG Xbox and Xbox 360 while Sigma are inferior versions(and Sigma 2 is awful while Black 2 is just Sigma 2 with gore and in Unreal Engine 5)
Where did you hear that Itagaki forced Team Ninja to delete the source code? There is almost nothing I have ever seen or heard that says anything of the sort and as far as I'm aware both him and Team Ninja have not commented publicly on any such matter. The reason they port the Sigma games over the originals is most likely just internal politics regarding Itagaki's explosive departure back in the day, some part of their court settlement forbidding further ports of the games (highly unlikely) or the most likely answer of Team Ninja preferring /their/ versions of the games as the definitive ones.
I'm gonna be honest with you, anyone telling you not to play Sigma 1 is being waaaaay too picky. There are people who will tell you to only play Ninja Gaiden Black as if it's the only version of 1 worth playing and it is a very silly, purist mindset. The original 2004 release is way cheaper ($10 as opposed to $40ish) and is still an amazing game that unless you've already played Black you won't even recognize the differences to know what you're missing out on.
Sigma is arguably just as good as Black but in its own ways. It removed some of the "puzzle" elements of the original, but most puzzles in NG:B boiled down to reading a sign that said "go collect this thing here" and the fetch quests felt more like padding than anything. Some underwater sections in the midgame were trimmed down and considering they're usually seen as the jankest/worst part of the original I can't say I miss their uncut versions. Some sections like chapter 2 were expanded upon, with a fight that was relegated to a cutscene now being an ultra hard optional encounter of sorts. The Rachel segments are certainly not my favorite and are ultimately my least favorite part of Sigma, but she's not terrible to play and her missions are pretty short at the end of the day. In terms of gameplay balance, most of the changes are so minor that only hardcore players will really notice.
Even if I was the biggest Ninja Gaiden Black stan on the planet I'd have to admit that if Black is a 10, Sigma is like a 9 or a 9.5 at worst. Plenty of people also prefer Sigma to Black so clearly that version has plenty of merit. If it is the only version you can play, absolutely try it out and you will not be disappointed. If you do end up disliking Sigma you are almost guaranteed to have disliked Black as well because the games are so similar. The only thing that would be a dealbreaker in Sigma but not in Black is if you absolutely hated playing as Rachel to the point that even her 3 short missions were just too much.
Sigma 2 on the other hand is definitely its own thing. I still think the game is fun, but it is undeniably a super different experience due to the way that enemy counts and enemy HP were handled. I've seen footage of the original running on Xenia (360 emulator), and it looked pretty damn solid, with some people even completing entire playthroughs via emulation.
This user posted their settings and build after competing a run.
