Market value: no idea. If I had to take a wild guess, maybe Rocket's Tyrantar from the Japanese VS set. I have a few various ex/LV. X/Prime cards, and a handful of Japanese promos/Vending series cards/VS cards, but I think Tyranitar is probably my most valuable.
Personal value: a few different cards. My Vending Series Dewgong (which is also my avatar) because it was my holy grail as a kid, back before I knew I could find Japanese cards on eBay, before I had my own eBay account, or even before I had the guts to ask my dad to buy something for me on eBay. Base Set Chansey because I never had one as a kid, and one of my best online friends bought me one for my 21st birthday. Base Set Charmeleon because it was my first exposure to the Pokemon series back in late 1998, when I got a copy of the card in my then-newest issue of Nintendo Power.