In a dictionary I was trying to find the superlative of 'right' but I couldn't; but I notice that using 'rightest' makes sense, isn't it? Or should we write 'the most' such as It is the rightest thing or It is the most right thing? Honestly, I have never used 'the most right' nor does it sound good to me.
Rightmost can be used to indicate that which is furthest to the right, but not to indicate that which is most correct.
Adjectives can be either gradable (like hot) or non-gradable (like married).
Right and dead are both non-gradable. There is no such thing as deader or deadest: likewise, there is no such thing as righter or rightest. You may occasionally hear people using these terms, but they are generally doing so for hyperbolic of humorous effect.
You can, however, be nearly right or nearly dead: maybe that would be a better approach when comparing the relative rightness of things. partially can also be used with right.
In the UK, it is also common to say too right, when you agree completely with somebody.