There really isn't a "rarest, most valuable card," as it really depends on what people will pay for it, though this one's up there. There were only 6 made.
A Japanese Tropical Mega Battle No. 1 Trainer card, of which there are nine, sold for nearly $4,500. The Tropical Tidal Wave Finalist card, won by the 15+ winner at Worlds this year, sold for over $3,500.
I can't find any info on whether or not this card ever sold, and for how much.