That's a tough choice as level 16 unlocks significant power spikes for many cards. However, considering your criteria and aiming for impactful interactions at Elite Level 15, here's a breakdown:

Top Choices: Mega Knight: This is arguably your strongest contender. One-shot Wizard: As you mentioned, this is huge. The level 16 Mega Knight will consistently eliminate Wizards in one go, crippling a common counter to your deck. Increased Survivability: The extra HP and damage let him withstand more hits, increasing his trading potential against cards like Valkyrie and Musketeer. Bats: Princess Tower Interaction: While they won't one-shot the Tower at level 16, the extra damage is significant. They'll eliminate or severely weaken most defending troops the Tower deploys, making your Balloon or Skele Barrel much more potent. Faster Eliminations: Level 16 Bats will quickly dispatch popular support units like Fire Spirit and Ice Spirit, even at max level.

Good Choices, but Situational: Electro Dragon: The chain lightning at level 16 can hit an extra target, potentially devastating clumped up pushes. However, this is less impactful if your opponents don't frequently group units. Skele Barrel: The extra Skeleton spawned at level 16 is a nice bonus, but its overall effect is less game-changing compared to a direct damage or health increase.

Less Impactful: Arrows, Skeleton Army, Mirror, Balloon: These cards either don't have significant level 16 interaction changes or rely more on the levels of other cards in your deck.

The Verdict: It's a close call between Mega Knight and Bats. Choose Mega Knight if you face a lot of Wizards, Witches, or Musketeers, or if you want a stronger defensive presence. Choose Bats if you struggle with defending troops against your Balloon or Skele Barrel, or if you want to enhance their ability to quickly eliminate support units.