Yeah... he owns everyone, but also himself.

Just analyzing the battle system from VII-IX (ATB is evaluatable comparatively) FFVIII is the strongest battle system, so Zell is the strongest fighter in FFVII-IX. This is all assuming maximised setups.

This is because at 180 speed FFVIII characters are about 4x faster then FFVII at 255. This means he gets at least three turns first, Hero+Aura=Instant Win. Also there is an ability in FFVIII to stop pre-emptive attacks so there's not trick to stop it, if final attack and revive it would take for ever but FFVII characters would never get a turn. If Final Attack combined with KoTR, well then I did say Hero didn't I? FFIX is pretty close to the speed of FFVII so that doesn't get any more of an advantage. This gives us a clear order of FFVIII on top, I will evaluate the ones with the best limits as best, Selphie's the end trick (lifting the disk tray) will not be allowed as it's out of gameplay, on average her limits are by far the worst so I will count her as weakest.

1. Zell (Armeggadon Fist)
2. Rinoa (Angel Wings+Meteor)
3. Squall (he and Irvine are pretty close but Lionheart beasts)
4. Irvine (fast shot>3)
5. Quistis (degenerator, Shockwave Pulsar)
6. Selphie (still an FFVIII character)

Next we compare the battle system of FFVII to FFIX. FFVII is much more powerful with Final Attacks, Phoenix, KoTR, you know. I'll give Cait Sith second to last, though he may not deserve such and Aeris is dead. The comparison is based on max power of limits.

7. Barret (18 hits)
8. Cloud (15-20 hits)
9. Cid (about 12 hits)
10. Yuffie (10 hits)
11. Red XIII (8 hits)
12. Tifa (7 hits)
13. Vincent (6 hits)
14. Cait Sith

And now FFIX, I don't remember, I should play this game again. Only beaten it thrice.

In storyline value there are clearly characters that are setup to look strong, again I'm sorry but I'm only evaluating 7-9:
Cloud (obviously the strongest from VII), Squall (no one else comes to his level within that game except Seifer, but he doesn't stay in the group), Zidane (main characters are shown as powerful, he's a genome), and BTW characters from X and X-2 aren't given much reason to be strong, now that the aeons and Auron are dead.

Cloud is infused with mako energy. But he didn't make SOLDIER, he did however defeat Sephiroth, mainly in the group but for storyline value alone. His story makes him out oddly, first weak with good build and then incredibly powerful.

Squall has by far the most training, he worked for a military organization and did missions. He defeated 3 sorceress' in a team, a sorceress has hugely more power then a human. He is also junctioned to a GF which we see can easily multiply his power by four battlewise, though for this we can't tell.

Zidane has training as a thief and actor. He is however an angel of death, this is impossible to quantify but I would say it definitely makes him stronger then a normal human.

Training:
1. Squall
2. Cloud
3. Zidane

Power Boost (mako, GF, genome):
1. Zidane
2. Squall/Cloud

I think it falls down to Squall and Zidane between which I'm not sure though I have a large personal preference for Squall.