I just did a single thorough playthrough, but Rikku ended up being my top 1-2 punch with Tidus, using Auron/Yuna rotating as the #3. I didn't do absolutely everything as I missed a few secret summons, but I did beat that one really tough optional dungeon at the end which can push a few characters to max out their sphere grid paths and move into others. Tidus for example because extremely powerful because the end of his connects to the end of Auron's, so you quickly get a whole bunch of high strength boosts. Also Haste is always awesome.
With Rikku it came down to beating that last dungeon. I got an item that let me attach death effect to a weapon and gave it to Rikku since she had the fastest attack, and it worked great. At the end of her sphere grid path I think she gets some Alchemy skill which doubles the effect of healing potions, which works great with those albhed potions which I belove restore 2000hp to the whole party and cure status alignment. So basically, just keep stealing them and you don't need Yuna for healing. Finally, once she gets to the end of her path you should have a teleport sphere or two so you can put her in the same place Tidus is, getting a ton of strength bonuses making her attacks do a lot of damage.
Now if you want someone useless that would be the cat guy, whatever his name was. Now he started off strong, having the pierce ability to attack armoured enemies, but before long you get Auron who replaces him in that role. The key problem though is his sphere grid ends too early in the game, and to branch out you have to choose one of (I think) Yuna/Rikku/Lulu's paths. This absolutely kills his physical attack because they don't get much strength bonuses, and while you can cross him into a magic class character he's too far behind the others too be useful, unless I guess you do a whole bunch of grinding just for him. Or maybe if you're using a strategy guide that can tell you how to get all the blue magic spells and what works best. But other than that it gets to the point where he just no longer has a role in the party so you stop using him. At least until that one part of the game where it forces you to use just him for a fight, which wasn't hard but took forever to beat.