All good athletes want to win and all athletes who want to win eat a well-balanced diet; therefore all athletes who do not eat a well-balanced diet are bad athletes.

The best conclusion from this statement is that

The correct option is: no athlete who does not eat well-balanced diet is a good athlete.

To understand why this is the best conclusion, we can break down the statements in the premise using formal logic:
1. "All good athletes want to win." Let us represent "good athlete" as $G$ and "athlete who wants to win" as $W$. This statement translates to: $G\Rightarrow W$ (If an athlete is good, then they want to win).
2. "All athletes who want to win eat a well-balanced diet." Let "eats a well-balanced diet" be represented as $D$. This statement translates to: $W\Rightarrow D$ (If an athlete wants to win, then they eat a well-balanced diet).

By applying the transitive property of logical implication, we can chain these two statements together:
Since $G\Rightarrow W$ and $W\Rightarrow D$, it logically follows that:
$G\Rightarrow D$ (All good athletes eat a well-balanced diet).

Now, let us find the contrapositive of this combined implication $G\Rightarrow D$. The contrapositive of any conditional statement "If P, then Q" is "If not Q, then not P", which is logically equivalent to the original statement.
The contrapositive of $G\Rightarrow D$ is:
$\neg D\Rightarrow \neg G$ (If an athlete does not eat a well-balanced diet, then they are not a good athlete).

This statement directly translates to: "no athlete who does not eat a well-balanced diet is a good athlete." Therefore, this is the most logically sound conclusion that can be drawn from the premises.