It depends solely on your playstyle and your level range, also normal/hard mode. If you exploited like other users and can one shot almost any monster with any skill , just pick something that works for you.
If you are close to finishing the game (all cards,etc) you will be around level 14/15.
I really like Titan Throw as it deals the most raw damage out of all skills ive tested on level 9. Easily crits for over 3k. If you combine it with Dash you can pick it up easily too. Whirlwind deals good damage and has strong knockback, making it a safe pick. Berserker works very nice when farming monsters, makes you one shot things and heavily increases attack speed, reducing attack delay = faster killing.
Smash is meh. The Damage is subpar, it relies heavily on proper positioning to make full use of it. In that time you could have easily killed all monsters with another skill much more easily.
Heroic Slam is a good basic move. Strong, good knockback, but limited range.