Best option is definetly a two point sling, as it offers the most versatility of the three types.
In general terms the ability to go hands free also allows you to operate binoculars, range finders, and nav gear much easier. Although all the sling types are designed to allow you to do stuff with both hands while maintaining retention of the weapon, they have different conceptions of what your doing with your hands, and for how long you have your hands off your weapon.
Single points are very good if the only time you anticipate needing to relequish control of your primary weapon is to transition to a secondary weapon for a very short duration of time. It gives up a lot of ability to control the weapon during any kind of dynamic movement or prolonged period where you need both hands in exchange for the ability to very rapidly transition to a secondary or to transition to your opposite shoulder for various tactical reasons. Additionally the only real place you can have the weapon slung with this style is muzzle straight down right on your center line, or if you fiddle with it under one of your armpits.
Bottom line is I would view a single point sling as a very mission specific piece of equipment.
Two points are as mentioned a very good do everything sling. You can very easily cinch down the BFG Vickers Sling mentioned above so as to have the weapon basically locked onto your body which is highly desired when your needing both hands to do another task, such as moving casaulties, ammo, climbing etc. With the weapons held so tightly to you your not going to have the same problems experianced with the other types of having your weapon smack you in the shins, head, face etc. That same ability to cinch it down can be used to really lock your weapon into a solid firing position in the prone, sitting and kneeling positions allowing you to engage to a farther distance than might be the case otherwise. Similarly to a single point it still allows you to transition to a secondary virtually as quickly. It does give up a bit of ease of use in transitioning to an alternate shoulder but its not unmanagble. Additionally the two point can have the weapon on your front, on your back, muzzle up, muzzle down, either is very easy and its very easy to go from one to the other.
Three points... Well as already mentioned above as well when paired with the M-16/AR-15 family of weapons they are just a pain. They are far more complicated than either of the other options with more webbing hanging off of everything. The biggest problem is that you have part of the sling running from the front signt post back to the pistol grip/butt stock area running right across the bolt catch and the charging handle. They do not support transitioning to anything whether it be a secondary or your other shoulder. Bottom line with a three point is don't, there are better options.