There are several pages on the web that have done thorough comparisons of different wifi antennas' performance.

• Gregory Rehm's tests
  • Pringles cans do almost as poorly as an omnidirectional antenna
  • soup-can waveguides do significantly better than Pringles cans
• Martin Bugs' tests
  • the parabolic reflector ("Confier") did the best
  • slotted wavegruides ("TM waveguide", "RC waveguide") came in second place
  • the biquads came in third place
  • a soup-can waveguide ("cantenna") of diameter 4" came in fourth place
  • a collinear omni came in fifth