Well width-wise (at the nut from edge to edge), I'd have to say Ritter 4 string basses have the thinnest width on a stock neck at 1.18", other than that, I've head that some of Jeordie White's (aka Twiggy Ramirez of Marilyn Manson) custom shopped Gibson basses have necks with width at the nut as slim as 1.0 inch. But then again anything is possible with custom shop, and I don't think most of us here can afford too many customs, so lets stick to the basics. Now as far as my experience, Ibanez Soundgear line of basses has hands down the thinnest neck depth wise (back of neck to front of fingerboard), don't think they were the thinnest necks ever made, but definitely the most comfortable thinnest necks I've played. According to Ibanez's website these basses have a neck depth of .19mm at the first fret which is pretty damn thin and perfect for fast transposing. The Fender Jazz necks are maybe a milimeter or 2 slimmer than the Ibanez SR at the nut, but definitely not thinner (depth wise), I own both and comparred them side by side with a measure tape and by putting them together face to face. For a 5 string, my favorite so far was the Music Man stingray 5, thinnest and slimmest neck I've played yet, although Ritter claims to have the thinnest necks of all stock made bass guitars (nut-width that is) for basses of 4 to 7 strings, and their specs do seem to support that info, never really held or played one, so can't justify from my experience here. But for the comfort and tone, defintely the Ibanez SRs are the best.