There's 4 top-tier defenses to e4, all of which are good enough up to the top level.
1...e5, the Double King's Pawn opening which leads to numerous variations such as the Ruy Lopez, Italian, King's Gambit, Scotch, Vienna, Petrov, and so forth. These are arguably the most classically principled and instructive defenses where you try to maintain equality and then edge your opponent out, but can get wildly tactical in the middlegame.
1... c5, the Sicilian Defense which is arguably the most complex and dynamic defense (and generally not recommended for beginners) which looks to immediately imbalance and complicate the position. However, play is not as intuitive and you have to be prepared to play various anti-open-Sicilian positions. Black often has to last through an initial onslaught by White, with good prospects for an endgame, but has plenty of hard-to-find tactical opportunities if White makes any mis-steps.
1... e6, the French Defense which is arguably the most directly aggressive and counter-attacking defense. The center is usually closed, with two competing pawn chains and maneuvering to outflank each other, with the center blown open at the right time, creating a nice blend of tactics and positional themes.
1... c6, The Caro-Kann, which is the arguably the most defensive choice, where you typically get a very solid pawn structure and easy piece development (although they may be pushed around a bit), wait for White to make a mistake, with a very good endgame if you play well.
All of those are great choices, and lead to different positions. On top of that, you have some more rarely played defenses such as the Pirc/Modern, Scandinavian, Alekhine, etc. which can also give you a playable position, which you may or may might not find as rewarding, depending on what you're looking for.
