for me Yuna is the strongest, the summons are very powerful and when i got the holy magic other characters just waited till the battle was over. Next two would be rikku and auron...

Well I don't know which could become the strongest character, but my strongest and my first to get to Lvl 99 was Rikku, the weakest to start off, she was my first to complete the entire sphere grid and I got her some awesome mods to break over the usual limit of HP/MP.

Rikku!!!!

Mod Edit: Your post is vague, please put more effort.

I never really had a stongest character, generally i used a mix between tidus, auron, wakka, and lulu.

kihmari and rikku were pretty much useless, and i only used yuna for her summons overdrives against bosses.

Like Boomkin, I used a mix of them all. Or at least I tried, amongst...eight(?) or so it gets a little hard sometimes. I find the strongest character is usually the person's favorite, which of course, tends to end up being the best one.

Mine was Auron, since he was just a badass and I loved him. I used him almost all the time when he was necessary, which was quite often with shell enemies or ones with higher HP, like Behemoths or higher end machina. Though I guess for most of the time Yuna was the best for the summons. Though they can be kind of a cheat, even with Seymour who takes them out in one round, just have them in Overdrives beforehand, use them all and he's basically dead using Yuna solo anyway. But by the end I had Auron doing max damage, same as normal attacks from the strong summons like Anima, so they're overdrives were a waste.

For me it was Wakka, especially towards the end of the Batlle Arena sidequest. I ended up depending heavily on Attack Reels to bail me out of trouble.