The most valuable Mew cards typically include 1st Edition holographic cards from early sets, Gold Star variants, Alt Art cards from modern sets, and promotional cards with limited distribution
The English-speaking market has its own exclusives occasionally (like prerelease stamped cards or cards in exclusive boxes), but Japans promotional tie-ins are more frequent
Whenever Underdark Beholder attacks, reveal cards from the top of your library until you reveal an instant, sorcery, or enchantment card with converted mana cost less than the number of eyestalk counters on Underdark Beholder
What competitive formats is Foundry Inspector played in
destroy cards on the field up to the number of cards discarded from your hand, and if you do, inflict 800 damage to your opponent for each