There are several different vectors on which to evaluate cordless drills. This is one of the reasons why it is difficult to decide which drill is the best cordless drill. Let's start with the most basic thing:
Build Quality
Lithium Drills
By "build quality" I mean whether the components of the drill hold up well over time and use. If the drill doesn't have a quality electric motor it may "burn up" when you use it for an extended period. Likewise the clutch, which allows the drill to provide varying levels of torque, may start to slip or just break completely. There are also some switches on a drill and you don't want these to wear out.
You may be able to determine the build quality of a drill (to some degree) by holding it in your hands. Things that "look cheap" usually are. But I believe the best way to determine the build quality is to read through several reviews online. Build quality is something that reveals itself over time and because of the Internet you can leverage other people's experience. The second factor in finding the best cordless drill is the:
Battery
Generally the battery accounts for a significant amount of the weight of a cordless drill. The newest battery technology is the lithium-ion battery. They store the most charge in the least amount of weight and so can contribute to a lighter drill or a drill that runs longer. The other technologies are NiMH (nickel metal hydride) or NiCd (nickel cadmium).
One thing you need to know about NiMH batteries are that they lose their charge over time when not in use. You can expect them to lose about 30% in a month. Neither NiCd nor lithium-ion batteries have this problem. But if you don't plan on leaving your drill on the shelf for weeks and then using it without charging it, this shouldn't be a problem. Other than self-discharge, NiMH batteries are ideal for cordless drills due to their low internal resistance. Lithium-ion is better in the weight to power category, which is important, but they are also more expensive, so you'll have to decide whether the extra power or missing weight is worth the cost. The last aspect I will address is:
Features
The best cordless drill will have to have at least the features we've come to expect, but not all of the "bells and whistles," especially if you don't need them. We've all come to expect a keyless chuck, which allows you to change bits with only your hands, without an extra tool. The best cordless drill needs to have a keyless chuck and one that works well, not allowing the bit to slip.
One nice feature is a clutch. A clutch allows you to set a maximum torque for your drill so you don't over-tighten screws and strip them. We expect most drills to have a clutch. Another nice feature is having more than one speed. This allows you to use the drill with either high speed and low torque or less speed and higher torque, which makes the drill more versatile.
Another nice thing we've started seeing in some newer drills is a built-in LED light. This comes in quite handy when you're working in low light. But it's not a must have. You'll also see a lot of cordless drill kits with two batteries. If you use the drill for more than just casual use, you'll want two batteries so you can keep working while the other battery is charging.