Philosophers usually spend their time evaluating arguments, but arguments link up to other arguments to form systems, and systems themselves come in families of systems ('schools' we could call them, without too much abuse of a term that is already used for something loosely like what is meant here) and it's an interesting question of how one can evaluate an entire philosophical system, and beyond that, an entire school of systems. One would, of course, want to avoid the temptation into which people often fall of assuming there is one line of evaluation ('Thomism is better than Scotism'; 'Analytic philosophy is better than continental philosophy'; or whatever else, for which the obvious next question is, 'For what sense of 'better', as measured by what standard?').
It's difficult to do this in more than a rough way, but it seems reasonable to identify both internal and external standards of evaluation.
Internal standards of evaluation are about how arguments and positions in the system relate to each other, and there seem to be two key issues here: consistency and tightness. Consistency is obvious, of course, but tightness of system seems often to be admired. Very few people are Spinozists, for instance, but many people admire how each part is linked to every other part by clear and definite steps -- there aren't parts that are there by mere speculation or guesswork or rough analogy. This contrasts with something like Romanticism, whose very nature guarantees that it is loose -- unsystematic, we might even say, although one can detect some rather elaborate system-building even in the most aphoristic Romantic. The Romantics are system-builders, undeniably, but one part relates to another part often only by analogy or by speculation; we are left with a bunch of fragments that clearly have connection to each other, but the connection is often loose and sometimes rather uncertain. Another example is comparing phenomenology and Thomism. When St. Edith Stein, who had studied under Husserl, started her study of Thomism, she found that it took quite a bit of adjustment, because in phenomenology one uses, over and over, the same method for everything, whereas St. Thomas just uses whatever method will make progress with the problem he's looking at. Because of its unity of method, phenomenology is tighter than Thomism.
People tend to assume, I think, that consistency and tightness go together, but this does not particularly seem to be true. Logical positivism is much tighter than ordinary language philosophy, but has consistency problems that the latter does not have. Locke is tighter than Novalis, but Novalis has a consistency of his own. Spinoza is much tighter than Leibniz, but arguably not more consistent; trying to catch Leibniz out in a contradiction is bold and likely to fail, and even if you succeeded it probably wouldn't much affect Leibniz's overall approach. Tightness of system makes consistency more shiningly clear; but it can also propagate inconsistency through the system. Most people are inconsistent sometimes; in a very tight system, inconsistency sometimes easily turns into inconsistency all the time. Looseness of connection isolates failures of consistency.
External standards can be of various kinds, but pragmatic value is one that comes up often -- how useful is it for some other field or area of human life? Obviously, the question of importance is 'useful for what', but if you ask these questions, you often get interesting results. For instance, most philosophers of mind today would probably, in the abstract, take usefulness to neuroscience to be a sign of quality for an account of mind. But if we ask what school in philosophy of mind has contributed most, and contributed the most important things, to neuroscience, there is no doubt whatsoever: the answer is substance dualism. Tell a materialist philosopher of mind that and he will think you are joking, so you have to grab them by the neck and drag them through the actual historical evidence, but there's no real room for doubt. Hylomorphists contributed a few things, particularly early on; materialists have supplemented, particularly in recent decades; but modern neuroscience owes its existence primarily to substance dualists from Descartes and Steno to Sherrington and Eccles. It was they who did the major early work on the structure of the brain, it was they who did most of the foundational work on nerves, neurons, and synapses. And it is not difficult to see why -- if you are a substance dualist, you take brain events to be related to mental events, but since you don't identify them, you are not under any pressure to interpret this brain event as that mental event, and can just follow the neural evidence wherever it goes. It allows the inquiry to have full importance while minimizing the temptation to pre-impose interpretations on the evidence. Substance dualism is structured in a nearly ideal way for the study of the brain, as Platonism is structured in a nearly ideal way for the study of mathematical structures, as Romanticism is structured in a nearly ideal way for thinking about artists. Depending on what you were doing, you could probably find a school that would be more useful for this or that particular purpose; but it would be hard to find one that was more flexibly useful for a wide variety of related purposes.
One reason all of this is interesting to consider is that it touches directly on the question of what one builds philosophical systems for -- what kinds of systematicity in philosophy are desirable, and why?
