We always use the article "the" when we talk about superlatives.
Superlatives are things on the very highest or lowest end of a spectrum, for example:
- He is the fastest man alive
- This is the coldest day ever
- This is the most tired I have felt in months.
Superlatives always end in "-est" or "-st" so they are easy to identify.
We must use "the" before each one because there can only be one, specific superlative for a given thing. There is only one "fastest man alive", there is only one, "best day of my life", etc.
In your example, it should be, "the best idea so far" because you use "best", which is a superlative. You are referring to one, specific idea that was the best.