Hi u/sergiothequick - it depends on your budget and whether you want an auto or manual lens.
Auto
Manual
The widest manual lens is the $499 Venus Laowa 7.5mm f/2 - but if $499 is too much of a stretch for your budget, I recommend the $199 SLR Magic 8mm f/4 instead.
This lens can produce results like this, mounted on the OG BMPCC:
If you really need the extra 0.5mm of focal length and 2 stops of light - and can afford the $499 price tag - get the Laowa. Otherwise, the SLR Magic 8mm f/4 is a a very good value for your money.
Hope this is helpful and good luck finding the right wide angle lens for your needs!