Which kind of harmony did classical era composers prefer?

Classical Era composers preferred a Diatonic Harmony, favoring a Tonic - Dominant - Tonic scheme (I-V-I)

Explanation:

The diatonic scale is formed by 7 consecutive musical notes that includes 5 whole tones and 2 half tones. Any sequence of seven successive natural notes, and any transposition of them, is a diatonic scale. In classical music, the diatonic scales are simplified into two variants: Major diatonic scale, and minor diatonic scale.

The tonic note is the note that defines the tone of the piece, it is the first note of a diatonic scale. The tonic center was the most important one that a composer used in a piece of music, with most pieces beginning and ending on the tonic, and usually harmonizing to the dominant (the fifth scale degree above the tonic) in between. It´s called the dominant because it is the second most important to the first scale degree, the tonic.