Top three most effective opening traps?


The most effective opening trap is getting intermediate players to study them, thus keeping them at that level forever.

1 PQ4 NKB3 2 PQB4 NB3 3 NQB3 PK4 4 PQ5 NK2 5 PK4 NN3 6 BQ3 BB4 7 KNK2? (aiming for a flank attack along the KB file. Correct is 7 NKB3 which allows better defense of White’s kingside.) 7...NN5! 8 O-O QR5! 9 PKR3 NxP 10 RxN QxR+ and Black is up the exchange.
Not very much a trap, but a very very common mistake that White makes in this position. Black does not have to make any bad moves to set up the position.
In actual, after 6...BB4 White is 3x most likely to play the natural-looking mistake 7 KNK2? compared to the correct NKB3, if you see the lichess players database!