Gunslinger - Of all the classes in the game, this is definitely one of them. There's just not a whole lot to say; it's just like an archer, but with guns - and since "archer" isn't a class, why would "gunslinger" be one? In practice, they deal decent but unspectacular amounts of damage (whereas actual archers get some of the highest DPR in the game), and they bring nothing else to the table. A clear example of Tier Five, and note that most weapon-using classes have a gun-wielding archetype, and almost all of those are better than the gunslinger class. Well, except the slayer and swashbuckler one, that is.
Shifter - One of the most maligned classes of the game, but bear in mind that it's derided because it is highly unoriginal, not because it's ineffective. On the one hand, there is little or no reason to play a shifter if the hunter or druid are available, or other shapeshifting archetypes. On the other hand, the shifter does get good damage numbers, and it has some decent abilities like early-access flight. All in all, it's much better than its reputation suggests, but "much better" still doesn't bring it above "adequate". Tier Four.
Exception: Oozemorph archetype is Tier 6 for being completely unplayable.
Slayer - It's utterly bland and forgettable, but it gets the job done. Its numbers are good, but it lacks stand-out abilities of other melee classes, such as fighter advanced weapon training or bloodrager bloodlines. Overall I'll give it Tier 4.5 for that.
Swashbuckler - Conceptually, it's unclear why this exists as a separate class, instead of a fighter with specific style, or perhaps a melee rogue. Mechanically, it gets a great ability at level one (parry & riposte), a good ability at level two (charmed life), and is entirely boring after that. So in practice, people don't play this as a full class, but take a one- or two-level dip on e.g. rogue or mesmerist or even barbarian (as parry & riposte works with any weapon and doesn't require dex). A hypothetical pure swashy brings decent-but-not-amazing damage to the table and doesn't really do anything else; and the fact that even casual players see this as only a dip makes it Tier Five.
I'm unable to find this specific exception on aonprd; it appears to have been errata'ed out.
Eldritch Archer.