here are my candiates for who tobi is
1.Older member of Uchiha or Uchiha Izuna. Izuna said to be madara equal. He should also be very knowlegable about the ninja world so knows about the sage of the six path. There is no solid proof he isn't dead so could be him.
2.Uchiha Shisui because i believe he is genjutsu master and can manipulate well as Tobi. His obsession for Genjutsu maybe led him to carry out the moon eye plan. Also very reasonable for him to participate in Uchiha massacre for the same reason as itachi and maybe to prevent them from interfering with his plan and learning about the Uchiha monument
3. Madara creation or someone using Madara's body. Since Tobi has some of madara's capabilities. It is assumed Tobi is a creation or someone else is using Madara's body or cells to perform Madara actions. Since tobi can use izanagi someone else might be in madara's or other Uchiha body while using harishama's cells.

i dont think shisui wud do sumthin like that and besides itachi saidhe killed himself or sumthink like that.

the older brother from the ridouku sennin story sounds like a perfect candidate as well

I also believed Tobi might a fragment or soul of madara, but as the series proceeds I believe less of it. Because Tobi action seem different and distinguishable from the real Madara. Tobi hides behind a mask while Madara attacks head on. There is no reason to hide behind the mask if he was madara.
I also thought about another theory relating to this, but little crazy. Before Madara died, with his rinnegan or eternal sharingan he was able to perform a genjutsu where part of his spirit or soul allowed to control the body and mind of another Uchiha while he was dead. So tobi is someone else but his memories and thoughts are of madara. Maybe this explains Tobi's split personality.

Uchiha shisui probably used his genjutsu to make it look like he died and deceived itachi. Plus, maybe he made it look like he died so noone would suspect him taking part in the Uchiha massacre.

Also older son of rikudo is not really good canditate because he is way too old to even exist. He should be using the rinnegan why the sharingan. Second, he doesn't need to conceal his identity no one knows how he looks. He should be able to capture all the nine-tails by himself because he is older son of the most powerful shinobi in ninja history. I don't see why he should hide behind that mask. Maybe tobi is descendent of the rikudo elder son born with the rinnegan where the rinnegan didn't yet diverge to sharingan and byukan. So he is like madara's ancestor or something.

I dnt think its shisui bcus if he was obsessed with genjutsu why wld he give itachi his only remaining ms bcus danzo took his other one. And someone that is interested in genjutsu wldnt give up that power. And tobi never said that "he tobi" took his brothers eyes he said that madara took his brothers eyes basically saying it in third person or as another person and as he said it he gripped his sleeve as though he was angry that itachi told sasuke that madara had taken izunas eyes and tobi said that izuna gave his eyes willingly. Although he cld have lied there and was actually hiding the fact that he was shocked that madra had taken his eyes so he may very well be izuna . Thats wat i think that is.

i agree shisui wouldn't have given his eyes if he was tobi. Also tobi was known to exist before when shisui was found to be dead so no need tobi to be shisui and cover tracks for uchiha massacre. i am also going with izuna as my best canditate because they don't show what really happend to him.
I don't believe when izuna gave up his eyes it impaired him so much that he died. If he was alive, What was he doing this whole time? Like how come itachi or someone else didn't notice him? Because itachi said he figured madara was alive why can't he figure izuna was alive since itachi knows alot and is a genius.

I think that it could be Izuna, Madara's brother. When he was fighting konan he had no reason to lie to her when he said that the rinnegan were his eyes first. and as we now know, the mangekyou turns to ems by taking a blood relatives eyes, then into the rinnegan. So madara's rinnegan would technically have originally been Izuna's.

Quoting kent0005

I also believed Tobi might a fragment or soul of madara, but as the series proceeds I believe less of it. Because Tobi action seem different and distinguishable from the real Madara. Tobi hides behind a

mask while Madara attacks head on. There is no reason to hide behind the mask if he was madara.
I also thought about another theory relating to this, but little crazy. Before Madara died, with his rinnegan or eternal sharingan he was able to perform a genjutsu where part of his spirit or soul allowed to control the body and mind of another Uchiha while he was dead. So tobi is someone else but his memories and thoughts are of madara. Maybe this explains Tobi's split personality.

what if the older brother sennin hides his identity anyways? like you said if it was madare he has no reason to coceal his identity so i think rite away that its not madara, an tobi uses a space time jutsu so i think its still possible its the older brother from the story

and nevermind that shisui idea cuz b4 he died he set that justsu on his eyes to "protect konoha" so i dnt htink shisui could b a candidate. izuna or older brother from ridouku sennin stroy