Which one is more appropriate to use: More or most?

Solution 1:

The 'most' is the superlative degree.

So it can't be used with 'than' which follows 'more' (the comparative degree).

Have a look at this: https://books.google.com/ngrams/graph?content=most+poisonous+than%2C+more+poisonous+than&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cmore%20poisonous%20than%3B%2Cc0.

It shows the only usage of 'more ... than'.

The sentence with 'most' can be like this:

"Some accounts claim that rattle snake was the most poisonous snake on Earth."

Solution 2:

Some accounts claim that the rattle snake is more venomous than any other snake that lives on Earth.

Some accounts claim that the rattle snake is the most venomous of any snake that lives on Earth.