Cheapest - Other than keeping your original and driving with in their limits -Scarebird.com sells a bracket and a parts list. If you carefully buy the parts this is the cheapest way to get disks.
Ever popular Granada swap, but unless you buy the new spindles with the correct geometry you could run into steering issues.
Most reliable - not sure what this means but if you can hunt down a set of stock disk brakes (it is possible) you get Ford engineering for a product designed for your car.
Most braking force - probably the kits that use the 13 inch rotors from modern mustangs e.g. the MustangSteve set up.

Be aware that changing from drums to disks may involve a change in wheels and tires which can get expensive.

If you search this site you will find over 300 posts on disk brake conversions. There are many vendors' kits available e.g. MP Power Brakes, Baer, Wilwood etc. so you have a lot of options.

Good luck

 

My Wilwood kit(they make good stuff for racing apps) was $612 for 4 piston aluminum Dynalite calipers, 11.75" x .810" vented steel alloy rotors, aluminum hubs that fit onto factory drum spindles with no modification, caliper mounting bracket, new bearings, hardware, and pads. You'd still need to get a master cylinder and a proportioning valve and flexible lines/fittings to go from the factory hardline to the calipers. They don't make flashy stuff or junk, this kit is somewhat plain jane lookin, but it works like a beast. They also have a large pad selection with compounds ranging from low dust quiet pads(what it comes with, Q pads) to superspeeday rotor eating racing pads(A and B pads). You also need at least 15" wheels to clear the calipers with this kit. With good tires my car will outstop just about anything on the road(helps to have a light car). And that's just one of many kits available if you choose to go that route. Trust me, if you like to go fast, GET BRAKES!!!! The factory drums suck for any kind of performance.