Or Nc3
What is the fifth best opening move?

g4 CHANGE MY MIND
This is your first post in this topic.
No one cares about you.
Go cry to your mommy about it.
That's a tough one. This is my personal opinion, so take this with a grain of salt.
Let's start with g3. I, personally, don't like playing it but I do find it quite tough to play against. The reason I don't like it is simple: I'd rather play the English. The point of the fianchetto is to attack the center, specifically d5, without putting pieces there. That's why you start with c4, to stop black from playing d5. But if you start this plan with g3, black immediately plays d5, taking the center. Sure, you can try attacking the pawn with c4, but black can always play c6 or e6. It's very easy to get into a passive position and get steamrolled.
Now, that's why I don't play g3. That doesn't mean it's a bad move. The big reason why people play it is either to transpose to Nf3, like you said, or to try a Hippopotamus or some sort of Double Fianchetto opening. If you like either of these, then I suppose g3 is a good way to get there. It's just not for me.
I have similar problems with b3. It doesn't do its job. Just like g3, it's trying to get one of the central squares, this time, e5. But, because it doesn't directly attack it, black just occupies the square with e5. And suddenly, the entire opening is refuted. Black takes the center, and white's fianchetto fails.
But with b3, that's not all there is to it. See, one of the reasons people like b3 is that you don't have to attack e5 to get some central control. Instead, you can use the fianchetto in an entirely different way and aim for c4, before or after d5. A common strategy I've seen is to attack e5 in a way that black has to move their pawn to d6, then play c4. I admit this is pretty tough to break through.
The good news about this for black, though, is that it isn't strategically devastating. In a position like the one I mentioned before, you can still gain space with f5 and dominate white's unsupported pieces. Additionally, the bishop on b2 isn't actually doing all that much. There are places it wants to be more. In fact, from black's point of view, you could even try an early e4 to try and stop the white plans before they even begin.
Overall, I think b3 is better than g3 as a general opening. But, there's a better option for white, that can actually do its intended job.
f4, or the bird's opening, is both inferior and superior to the english opening at the same time. By that I mean that both openings have a similar intention but different pros and cons. For example, I would argue that the bird is better at controlling e5 than the english is at controlling d5. The main reason for this, among other things, is that in the english c6 is a viable option for defending d5 while in the bird f6 is usually not. Due to this, the bird often leads to a knight on e5, which is very good for white. In these positions, white sometimes gets a huge advantage just from activity. They will usually fianchetto the bishop with b3, put a knight on f3, and sometimes even play Bb5 to get rid of some defenders of e5.
However, f4 is not without its weaknesses. There's the obvious problem of the From Gambit. Does this work well? The short answer is yes, it does. Black gets at least equality and at times more. But if you want to play the bird, you have to take this risk. And the good news is that in this dynamic opening, both sides have chances. So if you're good at that kind of thing, don't be too scared of the From gambit. But do be careful. Very careful.
Of course, these aren't all the openings you can play on move 1. But it's most of them, as the others are usually either passive or just unsound. My personal favorite is f4. Thank you for coming to my TedTalk.

This is my personal opinion, so take this with a grain of salt.
I'm on a sodium free diet, so I deleted the rest of the quote.
Thanks for the warning. I'll send you a trophy ❤️
This is my personal opinion, so take this with a grain of salt.
I'm on a sodium free diet, so I deleted the rest of the quote.
Thanks for the warning. I'll send you a trophy ❤️
Whoops, didn't realize my settings wouldn't let you do that. Fixed it I think

g4 CHANGE MY MIND
This is your first post in this topic.
No one cares about you.
Go cry to your mommy about it.

g4 CHANGE MY MIND
This is your first post in this topic.
No one cares about you.
Go cry to your mommy about it.
So did I change your mind or what?

g4 CHANGE MY MIND
This is your first post in this topic.
No one cares about you.
Go cry to your mommy about it.
So did I change your mind or what?
have you ever in the past few hours considered the possibility that I was not serious? I struggle to comprehend the possibility that you believe that I was serious.

Lol.
No, I don't take anyone on the forums seriously.
Less than 5 people probably, and I bet you couldn't even name one of them.

Nakamura likes b3 and plays it against strong opposition in blitz at chess.com.
But Naka doesn't play your b3, if you know what I mean...
... and as a gender fluid cockatoo I bet you do.

That guy is a recurring troll. He posted some transphobic stuff (his word not mine) yesterday and got banned, so now he's back. Just some dumb attention seeker. Nothing you say to that account matters, it will be closed (by him or chess.com) after the weekend is over.
b3