I will preface this by saying do what works for you.
I start nearly every build with the helmet. That's my personal choice as I take the view that if I decide I no longer want to do the suit at some point during the build at least I got a cool display piece out of it. I have several helmets that never got suits to go with them because I changed my mind on the full suit. I don't regret a single one of those helmets either. The downside is that if you're not experienced, and depending on the method of creation, they can be frustrating when they don't turn out exactly as you want them and can be more on the painful side to remake when you do have experience.
That said the other really great place to start in my view is the handplates if you're not experienced building yet. Handplates are small and tend to have relatively simplistic designs which are great for starting out. They don't tend to get a lot of scrutiny from other people compared to the helmet and chestplate, so "mistakes" are less risky. Also because they're small, remaking them after you've gained more experience and knowledge is less of a lift than re-doing a helmet.
My personal take though is to save the chest/back for nearer the end of the process. Outside of the helmet, it's typically the most scrutinized part of the build and tends to be on the more challenging side to build. Even if 3d printed in most cases you'd be printing in parts and want to have a process down that works for you for filling and hiding seams.
My build process tends to go helmet, boots, shins, knees, thighs, biceps, forearms, gloves, chest, back, undersuit, but I used to start with helmet then go arms, torso, legs.