Well, let's see:
(I'm just going to list them all, since all have their pros and cons)
The Wide Lens
This item boosts the accuracy of a move by 10% of the original accuracy. It is not much, but can make a difference.
For example, if you had the move DynamicPunch, which has 50% accuracy, the Wide Lens would increase that to 55%. If you had the move Hydro Pump, it would increase the move's accuracy by 10% of the base, so the 80% accuracy would become 88%.
The Zoom Lens
This item boosts the accuracy of a move by 20% if the holder moves after the foe. If you're playing Diamond or Pearl, then the holder also gets a boosted critical hit ratio when it moves after the foe. In Platinum, HeartGold and SoulSilver, and Black and White, then the holder does not get the critical hit boost (someone confirm?). In Double and Triple battles, the holder must be the last one to move in order to get the boosted accuracy. This works well on slow but powerful pokemon, such as Conkeldurr and Slaking.
The X Accuracy
This is a battle item only, which obviously means you can only use it in battle. It increases your accuracy in battle by 1 level, though I'm not sure how much that is. Its effects where off after the battle or when the Pokemon you used it on is switched out. In Pokemon Black and White link battles, there is a special feature that allows you to use items called the Launcher. The Launcher has a few exclusive battle items as well. These are the X Accuracy 2, 3, and 6. These items all boost your accuracy by 2, 3, and 6 levels respectively, but I'm still not sure as to how much that really is.
It's up to you to decide what you want to use for your Pokemon. Each one has its own pros and cons, so it really is your decision on which you want to use. The locations of these items can be found at BulbaPedia.