Arms.
Always a weak body part for me, mostly biceps. I'm a tricep and front delt presser so I've always had big tris and weak lagging bi's. Was stuck at 18.5 for a long time. Had never touched SEO until this offseason where I did Big A's protocol. Not exactly sure what my arms were pre-offseason, but I was at 20 20.5 unflexed now. I think Bi's are a pretty easy bp to hit too. Honestly I've thought about doing chest next but need to do more research and find more honest reviews on Syntherol on pecs. Front delt and tricep pressing have built my upper body well and I have a nice upper pec "shelf" haha but overall chest development is lagging now. With that, I've done a bunch more decline and dip work as well as really slowing the movements down to attack the mind muscle connection....that's the first approach...if that doesn't work THEN SEO.
I honestly think SEO is a last resort type of approach. Change the movement, change the speed, attack the mind muscle connection, change frequency and volume....once you've hit all of those and you're still a ways behind, try SEO.