Alright. It's true that the Archer has one major limitation in that he/she can only fire +3 arrows at best, and so not wound every single creature. You can partially get around this by carrying a sling and some +4 bullets around, leaving only the most beefed up characters immune to your attacks. In these cases, you'll have to resort to close combat weapons. I'll get back to that later.
Why is it the best class, you ask.
For that, you'll need to have defined 'best class', but we all get the general idea in it being the one class able to handle almost any situation, but mostly having the ability to kill armies of monsters at a time.
Let's compare the Archer to other classes to see what the kit lacks.
First of all, you do not have mage spells. An Archer on it's own is barely protected from an arsenal of powerful spells.
Second, the Archer has no ability to disarm traps, nor can he pick pockets or open locks.
Third, the Archer has some druid spells, but these are neither plentiful not powerful.
Finally, the Archer is not as good in close combat as most of the other warrior classes.
But, the Archer can certainly put up a good fight in close combat, better than most non-warriors anyway. It's hit dice are rolled for warriors too, giving him/her more hitpoints than the average cleric or thief.
More importantly, the Archer comes fully equipped with Hide in Shadows. In combination with a bow, this is extremely lethal.
First of all, you can scout out the area while invisible, seaking out the enemy. When you get yourself into a good position, shoot.
It takes only a little time to learn how many shots you need to take down a particular creature, so with a bit of practice you'll know when you can stay where you are and fire away, and when it is best to take some shots, dash around the corner, turn invisible and aproach the problem from a different angle.
This is the most basic and effective tactic there is for the Archer.
Imagine having a thief. You hide in the shadows, move toward your target, and backstab. Then you will likely have to run, because you won't last that long in close combat.
The Archer can do the same. From a distance (not as far to run), and, more importantly, without your oponnent taking a swing at you. It takes more time, because arrows don't hurt as much as a x7 backstab, but you can afford it.
In fact, time is one of your best allies.
Using the above tactic, you can safely take out anything, unless it has a missile weapon as well, or can cast spells.
If it has a missile weapon, you have a choice of several things.
First, you can try to shoot at the creature, then run away before he can return fire. Second, you can exchange arrows. Or, you hide in shadows and attack him up close. It depends on the terrain, and your character's proficiency and equipment which tactic is best.
With spells, things are even more fun.
Typically, you can fire arrows faster than the mage or cleric can cast spells. If he is sensitive to spell disruption, it's a matter of firing until he is dead. Otherwise, you'll have to be a bit more 'resourceful'.
Your worst problem will likely be stoneskin/iron skins. This is a matter of firing at the right times, which you yourself can create.
You will have noticed that being invisibe near a mage will often cause him to cast 'true sight'. First, trigger the stoneskin, then run and hide. Return and start firing as soon as he begins the casting to take away the spell.
Other obstacles may be protection from ... spells, rendering your weapon useless.
As an Archer, you have a choice of arrows, both magical and regular. Other than that, you can get around the spells by simply waiting until the effects wear off. In this way, you can get a mage or cleric to lose all protective spells, after which you have a clear way to shoot.
As for offensive magic, you have little more protection than anyone else. You can retreat a bit to stay away from area effects, or try to time the moment between the casting of a new spell and the firing of an arrow, but usually you will have the mage dead long before that time.
So as far as the fighting goes, you are relatively safe shooting from a distance, but you can also take some unavoidable blows due to your hitpoints.
You can go even further by getting the right magical items to help protect or speed you, maybe to deal more damage and things like that.
The Archer's strength depends on his ability to single out oponnents, take them down one at a time. If you get caught in a smaller room full of creatures, with no real way out, it'll become harder on you, but you are still a fighter.
On top of that, the Archer has some small advantages such as to hit and damage bonusses, the called shot and the priest spells. These can actually be rather useful, especially to dispel magic, remove invisibility, find traps and keep wizards from casting too many spells.
All in all, the Archer can take out a lot of enemies, and, just as important, do so in a cool way. You may think hiding is no good, but that's not 'really' what you're doing. Take a random action movie. See how the hero takes out the enemy one at a time, on his own, getting caught only once so he can learn the badguy's plans first hand? This is all that, and more.
On top of that, the Archer is, for me, perfect not only for powerplay but also for roleplay. I like the idea of a ranger, and when I think of a ranger I think of someone stalking through the forest, living on his own and taking down threats from a distance, hidden behind the leaves. It's an unseen killer, and that's just what you can be.
Yes, of course you can't take out those immune to some magical weapons,but you can either wait until the spell wears off, or charge in. Just because you can not specialise doesn't mean you can't fight.
Look at the thief and the cleric. Do they need grandmastery to hit? (well, sometimes they do
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You may not like the different, not so straightforward tactics, but that doesn't make the Archer less.
In my opinion, it simply is the best class