I don't think that any of us could say that Final Fantasy X wasn't at least an entertaining game, whether eveyone thinks its good or bad is their own opinion, of course. However, the weakest aspect of the game is the seemingly lack of main villain, oh there's plenty of badguys, but who really is the 'Main Villain" of Final Fantasy X? There were 4 character in the game that seem closest to being associated with being the feature villain of the game, and listed my thoughts on what does and does not make them villain material.
Sin
What makes him the villain:
For the majority of the game, Sin stands as the force of fear and power in the game, he is coveted by two of the other applicants on this like (Seymour and Yevon) and feared and hated by the masses of Spira. Sin has stood to lay waste and destruction for 1000 years, only offering respite when a summoner offers to kill one of their own guardians and then sacrifice themselves to offer a 'Calm' of normally 10 years before the cycle of fear and death begins again.
What makes him NOT the villain:
Despite the grandeur, I consider Sin to be the least likely to be considered the 'Main Villain' of Final Fantasy X. Even if he is the force of fear and death in Spira, in the end Sin itself is basically a mindless power, and is more a tool to be controlled by the other villains in the game. This puts him up there in the same category as the Void from FFV or Meteor from FFVII, sure, Sin has the power to be the villain, but lacks the mind or the motive, its just a weapon.
Seymour Guado
What makes him the villain: Seymour may very well be the most evil person in Spira itself, coming from a tragic past filled with prejudice and abandonment, he rose up to slay his own father for power, and shows no remorse in using people like pawns for his own ends. Even death itself was unable to stop him, as he lives on as an abomination, harboring the desire to bring salvation to all of Spira by killing everyone, slaying the world to make a Spira of the dead to rule over like a god.
What makes him NOT the villain: OK, to be honest Seymour has all the motive and making of the game's main villain, he is murderously insane, wants to ascend to godhood, however I really think he believes he's doing the right thing, making him even more twisted and dangerous. Sadly . . . Seymour suffers from the lack of power to meet his goal, he has all the makings of a great villain, a painful past to cloud his judgement, the motive to hurt millions, and even the mindset that what he is doing is right, but he doesn't have the force to stand up to claim that title, although he does try (and fail) to gain that power, he is more in the category of Xande from FFII or Kuja from FFIX, he's a major threat, but there's just something with more authority that him to make him the 'Main Villain.'
Jecht
What makes him the villain:
Jecht is a brash man who seems to act the main character's foil int he game, causing Tidus an almost obsessive compulsion to get back at his father for his neglectful attitude towards him. Not only that, but Jetch bolsters two horrible forms of power, not only being at the center of Sin itself, but harboring his own powerful body as the Final Summoning of the last calm, killing the very summoner he was meant to protect. As an additional nod towards the title of 'Main Villain' Jecht was recently chosen to represent Chaos for the game Final Fantasy Dissidia, above the other choices on the list here.
What makes him NOT the villain:
Simply enough, Jecht has no villainous motive, anything that he's done to be considered 'evil' was either not his fault, or was resolved by the end of the game. He resolved things with his son, Tidus, and it wasn't his fault he was harbored by Sin, as his body was possessed by Yu Yevon. Even in Dissidia it was stated that although he has a 'body of darkness' he had a 'heart of light'. Really, Jecht falls under the same cirsumstanes as Golbez from FFIV or Edea from FFVIII, he's not evil, he's just being used as a tool, not unlike Sin itself.
Yu Yevon
What makes him the villain:
Yu Yevon is the cause of all the death and dispair in Spira, he keeps the infinite loop going, sacrificing the entire city of Zanarkand and tranforming them into the horrible beast of Sin to lay waste for any and all who oppose him. He excels in false hope, he turns a summoner's sacrifice into a murder and turns the body of the weapon used against him into his own prisoner, all while an entire civilzation reveres him as a God and prays to him for a mercy that never comes.
What makes him NOT the villain:
That's all well and good, but Yu Yevon was probabaly the most anti-climatic part of the game itself. He doesn't even have a body, all he appears as is a glowing symbol on a floating rock, in a fight that may as well have been a CGI cut-scene as its practically scripted thanks to the auto-life every character just so happens to have. Yu Yevon may have been the root of all evil, but honestly he is more reminiscent of Euno from FFV or Jenova from FFVIII, he's just the source of the current evil, but not the Main Villain anymore. Granted this sort of 'source' theory also could lead back to Cloud of Darkness or Zeromus, but Yu Yevon definately loses his foothold as the 'Main Villain' of Final Fantasy X with his lack of intimidating appearence and the (practically) scripted final battle.
In conclusion, really, none of the villains of FFX seem to have all it takes to obvious rise above the others as the Main Villain. Sin as the power, but lacks any sort of sentience over its destructive actions. Seymour has all the motive and mindset of an evil villain, but ends up being more bark than bite. Jecht offers the feel of a 'Epic Final Boss Fight' with his Final Summon, but beyond that he really offers nothing in the way of a threat of evil, he willingly offered to killed to release Yu Yevon. And as for Yu Yevon, he gets built up as this end-all evil, the source of it all, but when the party finally meets him, he's this mindless, disembodied floating symbol in a pathetic scripted battle scene, losing any credibility as the final threat of the game. Honestly what I would have done differenty? I would have held off on the Seymour battle until AFTER Jecht's Final Aeon fight, then would have had Yu Yevon enter the spirit body of Seymour Guado, basically combining Seymour's evil motives and intentions with the legacy and power of Yu Yevon, making a real epic finaly battle. They could have even left in the part where you need to sacrifice all the Aeons first to make the final fight that much more difficult, but that simple change in roster would have led to a real final battle, as opposed to the false final fight in Jecht and the dissapointing scripted battle in Yu Yevon, with Seymour being killed off and sent before either of them even started.
But as I said, this is just my own opinion on the subject!