My favorite pack is probably the English Red Logo Fossil pack with Aerodactyl. There’s just something about the red letters on the brown pack, and Aerodactyl is among my top 20 favorite Pokémon as well. It’s one of just two sealed booster packs I own (the other being EX Sandstorm with Seviper, my favorite Pokémon 
).
Second favorite pack is Japanese Sky Legend. I own a sealed booster box, but the pack has the same artwork. (The pack is cropped though, so the booster box definitely looks even better.)
Third favorite I had to browse through all the English and Japanese booster packs for a moment. Especially the Japanese side has a lot of amazing artworks, but I’ll have to go with the Double Crisis one. Not only is it my favorite set, but I just realized that the booster pack contains a Seviper as well (thanks for letting me spend some money again lol):
And an honorable mention to the Japanese Sword pack artwork. It’s simple, but I like the pose and floor he’s standing on:
Greetz,
Quuador