I didn't even remember that Pikachu could learn Slash in Red. Did you mean Slam, which it learns naturally? I'll analyze all 3, though, for the sake of it.
Consider the effects, Swift has a Power of 60 with the ability to hit everything (and in the original Game Boy, this includes Pokemon who used Dig or Fly), while Slash has a Power of 70 with increased critical hit rate. Slam has a Power of 80 (I think) and can flinch, but has a poor accuracy at 75.
All are Normal attacks that use Pikachu's Attack stat, which isn't very good. So you shouldn't be relying on the actual damage power of these attacks in general situations. Most scenarios you should be using an attack like Thunderbolt that uses the higher Special stat, which will often out damage a critical Slash from the get-go and still have a chance to critical hit. Against enemies that defend well against Special attacks, or specifically Electric attacks, then you would service yourself better by swapping out Pikachu for a different, more physically oriented Pokemon.
Swift is the best choice of the 3. The difference in power is very small, which means it is even less given Pikachu's weak Attack stat. A weak power move is best used to conserve the PP of your strong moves in taking out weakened/easy foes, and the best kind is one that guarantees success like Swift. Slam and Slash both rely on luck (either in hitting with Slam, or critting with Slash). Not to mention, it combines well against Flying types that know Fly, since you will be able to hit them in the air, making it very versatile. Furthermore, it's always reliable as a backup against any enemy that ends up using Minimize, Double Team, Sand Attack, and similar moves that reduce accuracy.