Bettas, gouramis, and rams--because they are smart, curious fish with personalities, who go around the tank actually doing and noticing things. They are attracted to anything new in the tank and explore their surroundings constantly, testing out anything interesting they find. You can see them thinking. I adore my rainbows and tetras--they are beautiful--but mostly they just swim around.
When I built an aquabridge, it was easy to predict which fish would find it and use it. The gourami and the betta started exploring it immediately, and despite some initial trepidation crossed it within an hour of my putting it in the tanks. They LOVED it and would cross it ten or twenty times a day, and sometimes would just sit up in it--I think it gave them a new vantage point for the world. When I took out the bridge to make repairs, I swear the gourami was very upset to lose his connection to the other tank. He circled around the place it had been for a couple of days. The neon and glowlight tetras never used it at all.
