Takamiya_Shin
Pain is a Uzumaki, and is you can't remember they have signed a bloodline contract with the death god. also his eyes before the became the rinnigan were seeped in the Japanese mythology and with it becoming the rinnigan it now has Buddhism because of the 6 paths. having him would would surely peak a gods interest. Whitebeard is a no go the god slayer has the power of the water god thus can suppress him as a transmigrator he would have some knowledge of his weakness. Aizen as a soul reaper is ok but where he would be good at is information gathering and crowd control. tho tony as long as he brings jarvis would be able to easily find him and his masters location and enable you to deal with him faster
tho why is tony in the group if you are not going to tech worlds here's a thought, Halo, enemy group joins covenant to wipe out the humans in order to obtain bodies for idk blood, souls, zombies, food what ever team goes there tony can get a great boost your story is chaotic and hard to follow anyway so it won't matter much and if a new member joins the group hopefully it's not another useless filler character
Mhh so I would take aizen his Zanpaktu actually might guarantee that he wins. And even if he doesn't use this ability he still is a decent swordsmen and is great with kido and hakuda (afterall he never has shown this ability and still became captain and remember till the end we never got to see him use his bankai)