Let's start with the two bad endings.
BAD END 1: MICHAEL
Franklin calls Trevor, saying they've got to kill Michael so the government will get off their backs. He refuses, as he refuses to betray someone like he was betrayed.
The game is set up with Franklin acting as a stand-in for the player. Michael's end makes this abundantly clear. Michael talks about how he finally got his life back on track, and says things like "we did it!!!" and "it was all us!!!". Then you chase him through a construction site, which, admittedly, isn't the best. You reach the very top of the... structure, and Franklin tries really hard to justify his decision to Michael (himself really). Franklin tackles Michael off the thing, and has a change of heart at the last second and grabs his arm. The player must decide whether to help him up or just drop him. If you help him up, he headbutts Franklin and falls to his death. You made the shit decision, player.
BAD END 2: TREVOR
Franklin calls Michael, saying they've got to kill Trevor so the government will get off their backs. He agrees.
Trevor likes Franklin. He has so few friends, and most of his were, uh, "conditioned" to be such. Franklin and Michael are really the only people who stand by him of their own will. So when Franklin tells Trevor he needs to die, Trevor calls him a fucking Judas and drives away. See, even when faced by death at the hand of a friend, he refuses to betray said friend.
You chase Trevor through traffic to... a place, and Michael crashes him into an oil tank. Trevor falls out of his truck, and lies on the ground, coughing. He's covered in gasoline and near death. Naturally, you have to shoot him. However, no matter what you aim at, you always hit the gas on the ground and ignite Trevor, killing him in a very painful fashion. YOU'VE GOT TO LIVE WITH THIS, JUDAS
This is the one you guys got. Don't lie.
Basically, this is the one that best fits Franklin's character. He can't betray his homies. The other two endings are shit for not resolving any plot threads. This one resolves all of them, for better or worse. After the foundry shoot-out, you have to take out the game's primary antagonists and Stretch.
Stretch is just thrown in to tie up that loose end. He doesn't put up much of a fight (he's just a guy with a gun surrounded by Ballas), and he doesn't even have any dialogue. Likely, you'd forgotten all about him. The Stretch events were too far apart, and the whole Families vs. Ballas angle wasn't presented enough to make anyone who hasn't played San Andreas care.
The Chinese man needed more screen time, as well. His events also suffered from being too far apart, and the lack of a pre-fight or even being able to see his face at any point in the game made it anticlimactic.
The FIB man's killing was pretty cool. You get to do it on his TV show, and can even be on-camera when you do it. However, I feel that this would have been better suited to one of Lester's assassinations, and not for a primary antagonist. You just kill him and move onto the next guy.
The final guy, Devin Weston, is the best-executed of the three. He has, like, lines. And a confrontation.
This ending is canon, because it was obviously the one they put the most time into, the one they expected players to get, and the only one with concept art.
I don't think GTA V was given enough development time, or too much time was put into GTA Online. The story has odd pacing, and there's no real satisfaction in killing three of the primary antagonists. To be fair, though, GTA IV's pacing was also kind of bad, especially that middle section that dragged on forever.
It also felt like there should have been more heists, because you build up your crew's skills, and only get to use them, what? Four times? You can actually get away with only hiring a hacker once. Having more heists would justify the golf course being $150,000,000.
I hope that the inevitable expansion/sequel has better pacing, and isn't set in San Fierro or a modern version of Vice City. It would be cool to go to Las Venturas and rob casinos with our three favourite GTA V protagonists and Packie.