“I rather” is always incorrect. It’s not even a common mistake, and might indicate a non-English speaker.
Instead, say “I would rather” or contract it to “I’d rather”.
But also, be aware that rather can occur by itself when there is no subject, with the same meaning as “instead” or “conversely”. Example:
Instead, you should go to the farm
is equivalent to
Rather, you should go to the farm
This does not have the same meaning as “I’d rather you went to the farm”. I is not the subject of rather in the example above. The sentence indicates that, in a general sense, you should go to the farm.