No information at all about what the OP's goals are. "Best" depends upon the goals.
With as many times as this question has been asked, wondering - :search:
If you are serious and actually do want some real help, spend just a little more time and put some effort into the post. More likely to get replies with some "meat" to them.
Assuming a close to plug and play exchange:
Manual PCM.
Manual Transmission. Hint be sure to get a T45 with a VSS sensor.
Transmission cross member.
6 bolt flywheel
clutch
pilot bearing.
clutch cable
drive shaft
pedals
Manual transmission wiring harness.
Consider manaul steering column to get the manual style key safety switch.
hand held turner to disable PATS. Will be very handy to trouble shoot after.
Ford service CD (buy off of ebay). Will need the wiring diagrams.
Book mark car-part.com for local salvage yards in your area.
Get to know your local salvage yard operator.
Consider buying a "donor" car. Try copart.com for complete salvage cars.
Get a couple of Mustang mags (MM&FF). Several suppliers actually sell kits for this swap.
Are you doing the work yourself? Have you priced trading in the car for one with a manual transmission?