I don't have that much of value, except there are a few cards that are particularly memorable:
Potion Energy from Team Rocket. When I was 8 or something, someone traded me this and I think it looks awesome.
Dark Tyranitar (from Jessie theme deck). My first Pokemon product. I liked dual types because the background colours looked nice. However the foil is peeling off the card now
Latias delta from EX Delta Species. Although quite average condition (bent, scratched etc.) it was shiny and delta species which were things I really wanted when I got it.
Jumbo/non jumbo darkrais (spacial rend, roar of time). First jumbo card (*it's so big!*) I put the big one into a plastic sleeve (A4) to stop it from gathering dust on my desk
Fan/Frost/Heat Rotoms. They were shiny and out-of-main set (RT something), and seemed really special until recently when I found out they weren't worth much. Traded Heat Rotom for a promo Mewtwo EX a few days ago.
Absol G Lv.X. My first Lv.X: everyone remembers 'their first ___ card', right?
Articuno EX. My first EX (ditto)
Admittedly I have not much that's worth that much. I haven't bought many cards from post-BW after I quit for a while after HS.
Still looking to own a old ex, shining or crystal pokemon (quite unlikely seeing as I can't buy these cards cheaply anyway).