The Definitive Spherecasters Tierlist

Well, with the magic spheres done, time to move on to an even less controversial, and even more objective tier list of the spherecaster classes. Not the outsider classes that have sphere archetypes, though I will try and factor archetypes into it.

S - Extraordinary
Incanter - Almost every caster build wants at least one level in it. Some want 2, and a few wouldn't mind 3. But many "I'm literally just a caster," builds are very happy to simply pick Incanter all the way through, and get bonus talents and what not, at 2x the rate of a full caster.
Hedgewitch - Although only a 3/4 caster, it's just such a flexible (and ironically flavorful) class that it at least holds some consideration for the majority of SoP builds. Not everyone will actually pick it up, especially as it's least good as a 1-level dip. but not making a consideration of it...

A - Good
Soul Weaver - I actually put the class right up there with Incanter, in terms of raw capability. But there are substantially fewer builds that just absolutely must think about taking it. But still very good.
Eliciter - Are you playing a Mind caster? Is that kinda your thing? Then you want at least one level in this. Each level is less efficient than the last, with the hardest drop off being level 2. But it's still good, if that's really your thing.

B - Fine
Fey Adept: Are you doing illusions? You probably forgot this class exists, and just picked Incanter, but this class does exist, and does hypothetically help, particularly with it maintaining illusions without concentration. It's neither hyper focused, nor particularly flexible, nor too flavorful, but... it is a full caster, with some extra abilities.
Armorist / Mageknight - Do you want to hit things... but magically? Here you go. Have fun. (Some archetypes make them much more notable, but even base Mageknight is decent....at its job. Some archetypes are tied to blood sphere, and are laughable.)

C - Medoicre
Shifter - Did you want to hit things... but by first turning yourself inside out? Well, here you go. Apex Shifter is the default archetype, and you lose a lot by not picking it. It is incompatible with all others. Have fun.
Thaumaturge - Yeah, it's that Wilder-like class that no one actually remembers exists. It has full CL (plus a bit more), but half talents. It can *potentially* have disproportionate power at level 1.... and you'll probably want to swap out immediately thereafter. Even if you truly are a hyper specialist, you are going to want Eliciter or Incanter first for their additional talents and caster levels that are constant and non-variable.

F - Ew.
Symbiat. Some of the most recent archetypes (including my homebrew) did try and make something of it. They didn't succeed too well.
Elementalist. It does one thing (Destruction sphere), and not very well. Except for in a few archetypes. Why would you not just play Incanter and specialize in Destruction? Well, again, archetypes (Twinsoul Elementalist) can sometimes change the "optimality" of damage per class level. But it's hardly worth being a half caster in everything but Destruction sphere, and additionally being hobbled by half casting talent growth. It says a lot that despite all the archetypes it got early on, even the developers forgot it exists. And only a few are even worthwhile. The chasis is just so hard to make useful or fun.

The OG spherecasters really suffer from "Hedgewitch ate my lunch" syndrome. Fey Adept is a great example of this. Create Reality is a really powerful and cool class feature. So cool in fact, that it's basically Fey Adept's only meaningful class feature. Guess which class can outright just steal Fey Adept's main shtick, while also getting to be a Hedgewitch?

The idea behind Elementalist is that you get to have a really good chassis (d8, 3/4 BAB, all good saves, Evasion) while still being a Mid to Full caster. In exchange you basically don't get class features. Whether this is a good tradeoff when you could have just picked Hedgewitch is a question that's up to you.

I have a love-hate relationship with Mageknight. They are very strong, very fun and tactically interesting to play, and very boring to build. You pick the strongest Mystic Talents, pick the gish-Spheres, and then juggle spell-point and action economy while maneuvering around the battlefield. They also require no class investment - everything worthwhile about Mageknight can be done with just a 2 level dip, 3 if you want Stalwart (all the better to fill the next 18 levels with Hedgewitch). The Marshal Controller archetype is also pretty much just a straight upgrade, and a rather absurd power increase.

I'm surprised you didn't put Thaumaturge at F. They suck and feel terrible to build/play. Attempting to balance class features with talent paucity was way more common at the beginning of Spheres' life cycle, back before the developers figured out that people using Spheres wanted to actually, y'know, use Spheres.

Honestly, I'm surprised that I didn't put it in F tier as well. But it's not a strictly horrible class. I mean... I've had no actual builds that ever even really considered using it. But I'm not actively off put by the class. There's no real harsh feelings towards it. The most object of feelings. And I do honestly see the interesting idea of the Wilder. I liked playing them in 3.5 days, when psionics were allowed... which was even more uncommon than even Spheres are today. Internet is very good at scaring DMs into never letting things be tried out at their table.

And, yeah, agree with your assessments. I will throw out that Being a Destruction sphere specialist, to the exclusion of other spheres is such incredibly boring design. Like it's nice to have Destruction sphere. Even a few talents in it. But being a half caster for literally everything else (not to mention having no modern "you get full talents" archetype) is just so harsh. Maybe if you were literally playing Pokemon, then being Destruction specialized wouldn't feel too bad.

Huh, I've never gone that much into Hedgewitch - sounds like I should take a deeper look. It does seem to have a lot of options.

Never tried Soul Weaver either, but no real disagreements on S-B tier.

In (partial) defense of some other classes:

Shifter - It does its job well, and the Bestial Traits do add some power over what just any caster with Alteration would get. Full-caster for Alteration with mid-BAB? I'd say it's a better option than Incanter for the "shapeshifting combatant" type. Agreed that you almost certainly want to be Apex Shifter (although plain Shifter has the better capstone). Consider that in a game not using Advanced talents, Shifter is the only practical way to be shapeshifted 24/7. Personally A-tier because I like shapeshifters, but B tier is fair too.

Thaumaturge - For effects where a single casting is very important, Thaumaturge will make that casting stronger. And with Totem of the Darkest Aberration, one Thaumaturge can provide this to any other party members who need it. And with Soulfire Wielder, you lose the uncertainty (and there are ways to make the cost easier at mid-high levels). One Thaumaturge in a caster-heavy party makes that party noticeably stronger.

Symbiat - You can get full CL for certain spheres (Blood, Fate, Illusion, Mind, TK, War) via archetypes. So if you're playing a specialist in one of those spheres, Symbiat is a full caster with a lot of side benefits for free. Also, Int to AC with mid-BAB makes it a good dip for some gish types.

Wild Mage thaumaturge is a favorite of mine (speaking as someone who wrote a substantial percentage of the class's archetypes). Wild magic talent flexing helps.

I do think it would have helped for the specialist casters to get more bonus talents in their specialty, especially as the options expanded beyond the base book, but that ship sailed.