From my research the English opening (1.c4!) seems a good
first move in top level chess. Although the fundamental differences
between various opening moves are small (*), and a good GM
obviously also tries to look at what the opponent usually plays,
or knows (repertoire). Yet fundamentally, it also matters to know
how theory develops, for example:
1.c4 e5 (1...c6 leads to Slav) 2. Nc3 Nf6 3. g3! Bb4! (3...Bc5
4.Nf3! e4 5.Nd4 0-0 6.Bg2 Bxc3 7.dxc3 d6 8.0-0 Re8 9.Nc2
and White maintains a slight advantage.
Otherwise, after 1. c4 Nf6 2. d4 e6 (2...g6 leads to Gruenfeld
or Kings-Indian) 3. Nc3 (g3 also is possible) Bb4! 4. Qc2! 0-0
(5. e4!? d5 6. e5 Ne4 7. Bd3 c5 8. Nf3 cxd4 9. Nxd4 Nd7
10. Bf4 Ndc5 11. 0-0 Nxd3=) 5.Nf3! d5 (c5 6.dxc5 Na6 7.g3!
Nxc5 8,Bg2 b6 [...Nce4 9. 0-0 Nxc3 10. bxc3 Be7 11. e4 d6
12. e5 dxe5 13. Nxe5 Qc7 14. Qe2 Bd6 15. Re1 Bd7 16.Bg5!N]
9.0-0 Bxc3 10.Qxc3 Bb7 11.Rd1! Qe7 12.Qa3! +/=) 6.a3 Bxc3
7.Qxc3 dxc4 8.Qxc4 b6 9.Bg5 Ba6 10.Qa4 h6 11.Bh4 c5
12.dxc5 bxc5 13.Rc1 +=
And so on; and yes, this is a Nimzo-Indian :)
or Kings-Indian) 3. Nc3 (g3 also is possible) Bb4! 4. Qc2! 0-0
(5. e4!? d5 6. e5 Ne4 7. Bd3 c5 8. Nf3 cxd4 9. Nxd4 Nd7
10. Bf4 Ndc5 11. 0-0 Nxd3=) 5.Nf3! d5 (c5 6.dxc5 Na6 7.g3!
Nxc5 8,Bg2 b6 [...Nce4 9. 0-0 Nxc3 10. bxc3 Be7 11. e4 d6
12. e5 dxe5 13. Nxe5 Qc7 14. Qe2 Bd6 15. Re1 Bd7 16.Bg5!N]
9.0-0 Bxc3 10.Qxc3 Bb7 11.Rd1! Qe7 12.Qa3! +/=) 6.a3 Bxc3
7.Qxc3 dxc4 8.Qxc4 b6 9.Bg5 Ba6 10.Qa4 h6 11.Bh4 c5
12.dxc5 bxc5 13.Rc1 +=
And so on; and yes, this is a Nimzo-Indian :)
Alternatively, 1. c4 c5?! (Symmetrical English) 2. Nc3 Nc6 3. Nf3
g6 4. e3 Nf6 5. d4 cxd4 6. exd4 d5 7. cxd5 Nxd5 8. Qb3
Nxc3 9. Bc4 Nd5 10. Bxd5 e6 11. Bxc6+ 5 bxc6 12. O-O
Qd5 13. Be3 +/=
(*) in the end Black can always equalize but it is not always easy
PS fundamentally the moves 1.d4 and 1.Nf3 are just as good, after
1.d4 Black can also opt for the Slav defense; after a few moves
White as (super-) GM probably can best choose the 'Anti-Moscow'
gambit, where there still is some more theory to be found. And after
1.d4 Nf6 2.c4 e6 3.Nc3 we get the Nimzo-Indian again after 3...Bb4
(then 4.Qc2! is advised).
After 1.Nf3 (Reti) Black has several choices besides letting White
transpose to Slav, an interesting possibility now is to go for the
Queens-Indian, after 1..e6! 2.d4 Nf6 3.c4 b6 and if you know the
latest intricacies of this defense, it still can be a very solid defense.