what's the best way to find my father who I have not had contact with in about 25 years?

[Mick W]

if you knew where you last lived with him, (remember the address) you could check the voters list to see what names were on it for the address you lived at, you could also check utility companies (gas electric telephone) for names they supplied services to at that address 25 years ago, local businesses such as garages (gas station) may recall him if he had an account with them, he may even have been known to the local police, or check records at local employment offices, and any local credit agencies, they might even be a court record of him being order to pay your mother maintenance for you, which would identify him.

once you get a sir-name (last name) for him, check to make sure he's not deceased, before embarking on a time consuming search, with a last name and approximate age it narrows the field down considerably, unless of course its smith or brown,