The fastest bus is the Stagecoach 61, which takes on average around one hour and ten minutes. It runs every 30 mins Monday to Saturday daytimes, hourly on Sundays. It drops you off in Blackpool Town Centre on Market Street, near Talbot Square/North Pier and the shops.
It's quite a pleasant run through the suburbs of Preston onto the towns of Kirkham & Wesham and then Blackpool (coming in via Marton), passing through a few nice villages along the way. The single fare will be at least £5, perhaps up to £6. Not great considering a Lancashire Dayrider is £6.80, but that's often the case here with singles and dayriders.
The other option is the Stagecoach 68 running every 15 minutes, which takes the coastal route through Freckleton, Warton, Lytham St Annes and the South Shore of Blackpool. It takes around one hour and thirty minutes, but is better than the 61 if you want to be dropped off on the Central bit of the Promenade. Same prices.
There's also the Cumfybus/Coastal Coaches 75 (Fylde Villager), but that's not worth considering unless you're going somewhere along the route or you want to spend half the day on a bus.
The National Express coaches are the quickest and drop off at the new coach station, if you can call it that, near the Tower. They're scheduled to take around 50 minutes, but easily manage it in less. The downside being that they aren't all that frequent.