The short answer is that the easiest is the Fitech system, that said you need to understand the the fuel system will have to be changed out or modified to work with EFI regardless of what actual EFI system you decide to buy (more on that in a sec).
With the fuel system you have 3 options;
1. Modify what you have (external)
A. install a return line
B. install an inline pump
C. install in line filters
D. vent the tank
E. Install a O2 sensor
F. Install a fuel regulator
2. Modify what you have (internal)
A. Mount an in-tank pump
B. install a return line
C. install an inline filter
D. vent the tank
E. Install a O2 sensor
F. Install a fuel regulator
3. replace the fuel system with an EFI ready system
A. Get a Tanks Inc. tank with EFI pump and sending unit
B. Run the applicable fuel lines
C. install an in line filter
D. Vent the tank (the Tanks Inc. tank provides for this)
E. Install a O2 sensor
F. Install a fuel regulator
Which specific EFI system to purchase depends on several factors, primarily cost and desired control. The simplest systems only allow you to put in very basic information (engine size, idle RPM, etc) and the unit does the rest. These systems do not control spark, only fuel. There are more complicated systems which will allow you to tune the car with a laptop wherein you can control a lot of variables. Most of the more expensive and complicated systems control both fuel and spark (if you install a hall effect distributor).
If you are looking for the easiest to install and quickest to get working, I would recommend the Fitech Go Street, it costs about $800 and comes with an O2 sensor and plate to install it. It requires like 4 wires to be connected and once you do the basic setup, it self learns; basically it will get better the more you drive it. There are similar other systems like the Holley Sniper, MSD Atomic, etc.
I am running 3 EFI systems currently, I have a Fitech Go Street on my 52 Dodge PU which has a 56 Desoto 330 Ci Hemi, a Holley HP system on my 70 challenger (with a EFI 6 pack), and a FAST EFI system on my 73 Cuda 416 (also a EFI 6 pack). The Holley system uses a Hall Effect distributor and the HP ECU controls spark and fuel whereas the FAST EZ and Fitech do not, only fuel.
Some systems do not require a regulator, however IMO I would always run one. You will see some systems advertised as not needing a return line, here again, I would HIGHY recommend running one and you will see in the instructions of them all that while their system "may" work without a return line, they all recommend you do run one.
Converting to EFI seems daunting and a lot of work, in realty it isnt near as difficult as it appears. 90% of the work is in getting the fuel system set up. Luckily, Tanks Inc. has a bolt in solution (I am using 2 of their systems, one in my Challenger and one in my 52). I built my own fuel system for my Cuda, but to be completely honest, I suspect I will be going to a Tanks Inc. set up in that eventually.
As to cost, the fuel system all in is probably going to run about $700 ($250 for the tank, $250 for the pump, the rest for sending unit, fuel lines and filter). If you go with something like the Fitech, there is another $800 ($795 about everywhere). So, in the end you will be into it for around $1500-$1700 depending on what direction you go.
Hope this helps