The Aargh Page:

Not surpisingly, "argh" is much more frequent than any of the alternatives, and the items with fewer 'a's or 'r's are more frequent than their longer neighbors. However, there are high-frequency islands, even way out in the long-word planes. For example, "a¹⁷r²³gh" (17,23) occurs in 171 pages, even though if you change the number of 'a's or 'r's by one, it drops at least 20-fold. "A¹⁵r⁵gh" is almost 100 times more frequent than its neighbors.

Update: See also The Ahhhhhh Page: "There is a lot more ahhh'ing than argh'ing."