Taking a step back, I think that what what you want these slots to do re: cross-pollination of archetypes is more important than the quesiton of which card is best in a vaccuum. The Dowser and Wall are really combo pieces for a Ghostly Flicker
deck and will be 23rd cards in any other deck unless the cube is very low-powered. Talrand and Mystic just care about spell volume (though one slows the game down and the other speeds it up, plus having different risk/rewards ratios) whereas Niblis is (closest to) a tempo topend that rewards both casting spells and holding them up. Conjecture is a cool combo piece/value engine that rewards building towards a big turn. I'd start with figuring out which of these four categories (blink combo, spell volume, tempo, and general combo) you want to support and going from there.
It's also worth noting that Conjecture, Niblis, and Mystic are a clear cut above the rest on power level, as others have pointed out.