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.

they are often in theme and variations

the string with the finger instead of using the bow