First of all, buildings do not commit suicide. That's the ultimate smoking gun.
Other than that, everyone has different preferences as to what constitutes the best evidence. I haven't research much outside of the outlandish claim that no planes are involved. However, nowadays I'm seriously of the opinion that the evidence for there being no planes is so overwhelming that, this on its own, is evidence that 9/11 was an inside job of some kind. See for instance this, this and this.
To summarize, there are at least 6 outright problems with the idea that planes hit the buildings, and only two aspects which supports it (the videos of it and the three or so pieces of plane debris that were later found). The first and most important problem for me is the fact that most live witnesses simply didn't see, hear or report a plane at all. People assume that everyone saw and heard the plane but those who were there and reported live simply didn't. That's just ridiculous, it would sound like being a front-row banger at a rock concert to everyone in lower Manhattan as the plane approached. If you were outside you would want to cover your ears. And if I've understood it correctly, the supposed planes entered at opposite sides of the buildings (one from the north, one from the south). Everyone on the street should have seen at least one of the planes, and everyone should have heard both.
The second is the argument that you can see in the videos that they are faked for various reasons. I won't comment on that as I'm not a video analyst, but it does seem shady in a lot of cases. Planes appearing out of nowhere, different descent paths, different backdrops in the sky on a sunny day with blue sky (is that a thing?), etc.
The third argument is the lack of vortexes in the smoke after the supposed planes hit the buildings. The smoke from the explosions should have curled up instantly and spun like crazy, and yet behaved as if there was no real wind turbulence up there.
Fourthly, a Boeing 767-200 can't fly at 590 mph at such a low altitude. As far as I've understood it, it's just impossible. In fact, even 390 mph seems to be pushing it. The recent crash of flight 4U 9525 for instance, the German pilot who committed suicide with a plane, crashed into the alps at 430 mph diving straight down, and that landing was at 2000m. That's 5 times higher up than on 9/11, as the roofs of the towers were about 415m high, but the explosions were below the 100 out of 110 floors. And as can be seen in the diagram, the descent speed slowed down the closer he got to the ground.
And fifth, aluminum planes do not enter steel buildings like a road runner cartoon enters a rock, leaving a perfect entry behind. They mostly telescope in on themselves, shatter completely and fall off. Some of it will of course enter a little here and there, but not full penetration instantly as if there's zero resistance. It's literally as if a Boeing plane flew into an artillery tank and just exploded inside it.
Sixth, once the explosion has died down, there is exactly zero evidence of plane debris in the footage of the hole in the buildings or below them. The entire plane was swallowed by the building, apparently. Chew chew.