Answering an Ancient Question, like the fossil I am. I just can't ignore this.
First of all...
We need both of the stat.
Higher Speed will allow you to attack clean, neat and first without any specialties.
Higher Attack/Special Attack will allow you to deal more damage, if ever you hit first or not.
But. Without attack, you can't deal much damage. despite striking first. ending up you're throwing snowballs over a mountain.
And. Without speed, you might die even before you can attack, considering the number of truck-that-moves-like-a-racecar in the higher tiers.
And as mentioned, having both mean you'll strike fast and deal lots of damage--It's best to have both, right?
But... let's take an example.
Ninjask - Has 150 base speed as well as Speed Boost (probably one of the fastest Pokemon alive.) and has a slightly-above average 90 base attack and deathly frail defenses.
What kind of moveset do people commonly use with it? Answer? Baton Pass/Protect/Swords Dance
As Ninjask's Attack isn't enough despite the godly speed and access to Sword's Dance.
Another one...
Emboar - Has 120 Base Attack, 100 Base Special Attack. Where's the speed? Down to 65 that is. Only it's HP saved it, it has frail defenses and has no priority moves at all, Flame Charge hardly gives beans.
What kind of moveset do people commonly use with it? Answer? Reckless, and no-one uses it, infact, like Rampardos, who has the similar issue, but worse.
As they say: "Emboar might die in the first turn anyways, so why not go full power?"
Looking at those--check the ones who has high in both.
E.g. Salamence
Decent 100 Speed, 135 Attack and 110 Sp.Atk
Movesets with Salamence are also decent. Accounting the fact it can outspeed slower Pokemon unlike Rampardos as well as hit hard unlike Ninjask and the newly introduced Regieleki.
Another example: Marshadow
High 125 Speed, 125 Attack.
Marshadow's moveset include a various options of offense, and might as well include Shadow Sneak, which is priority (despite being fast, because, wynaut strike faster?) and that crazy movepool of wide coverage. Marshadow hits hard enough as well as outspeed enough enemies, that's why it's loved by many.
IF your Pokemon is slow and can receive enough damage, that's good enough, but it's risky, compared to not allowing your opponent to move and just make them faint clean. Ehem Calyrex-Ice v.s. Calyrex-Ghost
- Priority moves DOES exist. e.g. Extremespeed/Quick Attack. These two moves will strike before any other moves with the lower priority bracket. Unless your enemy also used one, it won't be a problem to strike first. e.g. Scizor using Bullet Punch.
- Check this sites' page about Moves for more information.
And yet again, I'm saying here, both stats are important, and you must invest with both. That's why only a few Pokemon are in the better tiers as they're the ones who met either both stats or has high enough defenses to wall/survive (like Metagross). This narrows down the choices of which Pokemon to use to sweep.
Wait up, someone mentioned "Trick Room"!!!
What is Trick Room? - The move called Trick Room reverses speed for a number of turns.
This is useful for slowpokes, but is pretty much a ton of job than just go straight forward truck-that-moves-like-a-racecar. It may also not be viable as it consumes a turn, and moves like Taunt can prevent it as people commonly use a lead carrying taunt--and worse, your Pokemon supposed to set it up will die with OHKO from a truck-that-moves-like-a-racecar Pokemon ehem, Mega Alakazam Psychic/Deoxys-A Psycho Boost.
I hope this helps, I answered this at my bedtime. Ah yes, that brain activity during the night.