c4
e4 or d4

"1...e4. Best by test" - Bobby Fischer.
According to Stockfish, both are equal, but I know AlphaZero quite liked the imbalances it could create with d4 positions.

I've played e4 all my life. Except for two years when I tried playing d4. More or less it was all fine but I had a big problem playing against KID.

e4 has a lot of theory behind it that you would need to know(Sicilian, Spanish, Italian, etc). d4 also has a lot of theory, but you can play the London, which is a solid opening system that you can play the same thing to just about anything your opponent plays.

Personally I say d4... It's usually a question of whether you're a positional (d4) or tactical (e4) player, and I'm more of a positional player. But generally modern chess has been leaning more toward d4... because people are so booked up nowdays, and in e4 there are alot of very forcing lines, it's not as easy to make some uncommon positional move and get the opponent out of book. Also the berlin changed the way people evaluate going into the ruy lopez. Then you have to face the sicilian, which... is a ton of theory and black still gets a great game / scores just about even in many variations. Fischer said best by test a very long time ago.

@18
1 e4 is still most played at top level.
By about 9%, though from the data I've seen the games aren't filtered by time control... and the 17% of c4 / Nf3 games often transpose into d4 or pseudo-d4 positions but we don't count that, if we did I think d4 games would exceed e4 actually. But for the types of positions players prefer at the top level we'd have to say they go for positional games more frequently than tactical ones.
Though I actually don't think this matters too much, I don't think polling of master players is necessarily a good way to answer this question. If you were to ask the very best in the world what they thought that'd be more interesting. Though you'd still get a variety of opinions and alot of it would come down to individual preference. In any case, you do need a way to get out of book regardless of the move, and ultimately I think the type of player you are matters alot more.
Besides... for this conversation we should really be considering club level play.
I do e4 but what does anyone do the most