If Aristotle was opposed to democracy as an ideal form of government, what kind of government did he consider the best?
Aristotle classified states according to two variables: who holds power? And: in whose interest is it exercised? There are three politically possible answers to the first question (one, some and all:the kingship, aristocracy, and politeia), and two politically possible answers to the second (the holder of power, and everyone). Aristotle treats kingship and aristocracy as an ideal constitution run by morally and practically virtuous people and aiming at the development and exercise of virtue.
The ideal is the state in which the best, who are inevitably few in number, exercise power in the interests of all. However, since that ideal is hard to achieve, and even harder to sustain, Aristotle advocated a form of mixed government, or "politeia", in which all citizens "rule and are ruled by turn", and power is monopolized by no particular class. Aristotle was a vigorous critic of democracy.
I am not so sure that it is entirely correct to conclude that, "Aristotle was a vigorous critic of democracy". In an earlier response to a question regarding the different types of Government advocated by Plato and Aristotle, I stated that Aristotle was cautiously supportive of Democracy, due to its fragile nature and how it can potentially degenerate into Anarchy, if improperly governed. However, Aristotle's warning of Democracy's fragile nature and potentiality for Anarchy did not necessarily make him "a vigorous critic of democracy".
Your concluding quotation is probably better applied to someone, such as Plato-(and perhaps Socrates), who absolutely hated Democracy and was very much THE "vigorous critic of democracy", as evidenced in his two political works, "The Republic" and "The Laws". Aristotle's "Politics", as well as his uncynically objective analyses of various Constitutions, was not exactly the sign of someone who "vigorously" critiqued or opposed Democracy.
Remember Aristotle's biography. While Aristotle only lived 62 years, he spent much of his life under Monarchial rule both in Northern Greece and Athens-(under the imperialistic rule of King Philip of Macedon). However, Aristotle did get the chance to live in a Democratic Athens during its final years-(i.e. the time of Demosthenes), when studying at Plato's Academy. He would have seen firsthand how Athenian Democracy had functioned and it perhaps would have given him a more realistic understanding of how citizen based systems can be effectively successful...again, if governed properly.
I agree that Aristotle certainly did not lionize Democracy or Representative Democracy-(i.e. His "Politea"), but he was certainly NOT, "a vigorous critic of Democracy".