To preface this message I must introduce myself, and try and not sound pretentious in doing so, I am Scott and have been a memeber of the pokegym forums for awhile now, and I am one of the top collectors of pokemon cards. Some of the cards I have are the 97 no 1, 3 trainers, the secret super battle trainers, bi lingual stadium, victory rings, orbs, mysterious pearl/diamond, university magikarp trophy khan, and loads others. Anyway the reason I am stating this is because the information I will give about the rarest cards is NOT just an insert or estimate as you see on bulbapedia, they are from the people I have met over the years through the many transactions I have done, and are 100% legitimate.
Alright the pre raichu is not the rarest pokemon card, I know someone who sold it to the largest english collector in the world recently. The most sought after or expensive card is by far the pokemon illustrator and nothing else is even close. The No. 1 Trainer from 1997 has only 2 copies, there is a difference in the text of the 97, 98, and 99 card, and there were only two prints of the 97 set. Also others that are up there are the SSB and Tropical mega battle no. 1,2,3 trainers.
Anyway the pokemon illustrator will reach sums of 20k, even right now in this economy. I know of two that are waiting to be sold privately once the buyer reaches the agreed sum of 25k each. The pre raichu recently sold for 10.5k to the largest english collector in the world. I do have pictures of these cards and love to discuss anything related to trophy cards or serious collecting, so if you would like any more information I would be very glad to discuss anything related to collecting.