It is a fun question - and a measure of how often you visit WikiTree as compared to other W sites.
Like Dale I have WikiTree bookmarked and faved - but for some reason I usually visit Riksarkivet, where I often go for sources, by typing the adress. I don't think anything comes up with just one letter, but I'm down to "ri".
P.s. The reason for using this path in this case is probably that I get a convenient choice of subpages this way.