Answer: My least favorite is "you know." In any case, in Spanish those "filler" words are called muletillas (or, less commonly, palabras de relleno) and are very common. But Spanish speakers tend not to use one-syllable utterances as much as in English. Instead, they tend to use common words like este (usually pronounced as esteeeee, depending on how nervous the person is), esto (or estoooo) or in Mexico o sea (which roughly means "I mean"). Che is often heard in Argentina. In other areas you may hear es decir (meaning, roughly, "that is to say"). The "er" has its equivalent in the sound "eeeehh," and em is similar to the English "ummm."
Also, it is very common to use pues, which has a variety of meanings. Pues can be used at the start of a sentence as a kind of filler while you can get your thoughts together. Or try a ver, which is to say, "let's see."