It depends how "best" is defined, but a bus from Bolton to Bury may be considered the best unless you really dislike buses.
First 471 appears to be typically every 10 minutes with a journey time of around 30 minutes.
North West Traveline has bus times: htt:)p://www.traveline-northwest.co.uk/journeyplanner/
You can buy a day return Preston - Bolton and a Bolton PlusBus ticket in one transaction at Preston station or on some online sites.
Preston - Bolton
Anytime Day Return £10.70 (Railcard £7.05)
Off-Peak Day Return £9.50 (Railcard £6.25) [Mon-Fri not before 0930]
A Bolton PLUSBUS ticket gives you unlimited bus travel on participating operators services, around the whole urban area of Bolton and the 'Greater Manchester' region - see map.
Day tickets £3.50 (Railcard £2.30)