A
very open ended question.
It really depends on context. Monospaced fonts are more readable for tabular data. Hard to read fonts are thought to increase memorization of the subject. Using italics, bold etc. in the right context will also increase readabilty irrespective of the font. Then you have fonts that are designed for more fluid reading like Dyslexie which is thought to increase comprehension for people with dyslexia.
You really need to define what you mean with "readable" - it it comprehension or memorization you are referring to or that reading it is fluid (think of a telepromper for a presenter)?
