I'll reply to this by saying that "easiest" is a bit slippery of a term. In general, what makes the fugues of Bach (in the WTC) difficult IMO are the muscal and structural complexities, especially when played on a piano classes
, which allows for the performer to also be conductor to the voices, managing their levels of prominence, allowing for distinct traits for each voice, not just each hand. If we accept this as a basic premise, I would have to say that the Fugue in E-minor of Bk 1, which is uncharacteristically only in two voices, is your best candidate as it is really more an (2-Part) Invention than a fugue, so would represent earlier stages of development of the musician. If you wanted a bona fide
fugue that isn't too difficult, I would recommend the G Minor of Bk 1. It also has a lovely Prelude paired with it.
Let us know what you try!
That is one of the easiest Fugue that I know. But you can also try the Fugue from Prelude and Fugue No. 2 in C Minor (WTC I), which is considered one of the easier three voice fugues.