Slayer is a good choice if you really need a Full BAB progression but don't want to take Fighter for whatever reason. The Sneak Dice offer good damage and the bonus feats give you a lot of versatility.
Ultimately if you aren't just dipping, other classes offer more bang for the buck, but if you're just interested in, say, a 2- to 6-level dip, Slayer offers quite good value.
Fighter by comparison has slightly less damage, and because it isn't implemented correctly there's some wonkiness with how valuable the abilities are.
Ranger is much better kept pure to increase animal companion potency and get more spells. It's a much weaker class to dip in and out of. Freebooter is kind of an exception to this though since 4 levels of that gets you some really nice stuff.
Vivisectionist is also a good dip class but it's abilities are a lot more neutered by the low class level - your mutagen is shorter, your spells are less potent and less numerous. Vivi is much better if you get at least 6 levels or more, while Slayer is pretty good even if you only dip into it a bit. It's also medium BAB.