I pulled the recent games of two groups of top online players and looked only at their Black results: the top 50 of the Chess.com blitz leaderboard (6,843 blitz games, mostly 3+0) and the top 100 titled players on Lichess (13,865 games, mostly bullet and blitz). Score = points per game for the top player.
Their Black averages: 67% on Chess.com, 62% on Lichess, so compare each defence with that, not with 50%.
Against 1.e4 (Chess.com top 50):
- Sicilian: 585 of 1,606 games, 69%
- 1...e5: 451 games, 66%
- French: 176 games, 70%
- Caro-Kann: 124 games, 68%
- Scandinavian: 80 games, 68%
- Pirc (1...d6): 35 games, 56%
Against 1.d4: 1...Nf6 in 668 of 1,212 games (63%), 1...d5 in 322 (64%). Both a few points under their Black average, so 1.d4 was the tougher first move to meet.
Things that surprised me:
- The Modern (1...g6) was the only defence above the Black average on both sites: 74% in 78 games on Chess.com, 66% in 289 on Lichess.
- The King's Indian scored below average on both (60% and 57%).
- The defence White struggled against most on Chess.com was the Slav: the top players scored only 58% with White against it (64 games) vs their 73% White average. On Lichess it was the Alekhine (59% in 131 vs 67%).
Small samples, so 2-3 points either way is noise. Full tables for both sites are here: https://openingspy.com/guides/best-chess-openings-for-black
What do you play with Black, and does it match your results?
"mostly 3+0"
* the outcome of a blitz or bullet game has almost nothing to do with the opening.