I am a staunch advocate of "less is more." My observation is that at every level of the game, players are using too many conventions. Too often I see players (from beginner to world champion) misusing or forgetting their methods. Everyone would benefit if they would just KISS. (Keep It Simple, Stupid).
If I had to choose just 4 conventions, this would be my list:
1. Negative Doubles
4. Jacoby transfers
If you forced me to choose 4 more, this would be the next group:
5. 4th Suit GF
6. Methods (I prefer "DONT" ) vs. Opponents' Notrump
7. Weak jumps in competition
8. 2NT ask after our Weak 2-bids
If you insisted I fill my convention card with 12 conventions, these would be the next four:
C) Unusual notrump/Michaels, New Minor Checkback, Jacoby 2NT, Support Double
And, since people just love to learn/play/teach conventions, these would be my D-list -- choices 13-16:
D) lebensohl, Drury, Inverted Minors, Splinter Bids
Honestly, I think it would be counterproductive for 99% of players to go any further than this. The memory strain is not worth it. Someone once said, "The Road to Hell is Paved with Good Conventions."
This list is constantly referred to in "Talking Bridge."
updated: June 2012