So Bet on tails. On a fresh, unworn, clean penny the odds may be 80:20 in your favor. This obviously doesn't apply to a coin flip, however.
The answer is that the side with Lincoln’s head on it is a bit heavier than the flip side, causing the coin’s center of mass to lie slightly toward heads. The spinning coin tends to fall toward the heavier side more often, leading to a pronounced number of extra “tails” results when it finally comes to rest.