1. She is most suitable for the position.

1-2. She is the most suitable for the position.

(Are both the same in meaning? Are both grammatical?)

2. She is fittest for the position.
2-2. She is the fittest for the position.
(What about this one? Are both the same in meaning? Which one is correct?)

1 means she is very suitable ...

1-2 means she's more suitable than anyone else.

2-2 is the correct one between these two sentences.