Turkish is commonly cited as an example of a language which is, with only one or two quirky exceptions, exclusively suffixing. Cross-linguistically, suffixing is much commoner than prefixing and I have heard that no language is as prefixing to the same degree that Turkish is suffixing. I am aware that there are various languages (e.g. Russian, Swahili, Navajo, Nahuatl, Kuot) that do make relatively extensive use of prefixes but does anyone have any idea which languages are the most prefix-heavy? (is it perhaps one of the aforementioned?)
Athabaskan languages would be the "most prefixing", in (a) being almost or in fact exclusively prefixing and (b) allowing many prefixes (11 positions). Papers on Navaho include this, as well as J. Kari Navajo Verb Prefix Phonology and Young & Morgan The Navajo Language. One can check information from the related language Sekani, and it seems that the language Slavey (K. Rice, A Grammar of Slave) has over a dozen prefix positions. Another contender is related Tlingit, which however has more suffixes.