KEH, without a doubt. Go for "bargain" grade on everything but film backs. The grading is consistent, but film backs can be finicky and you're better off with an "excellent" grade one. KEH's bargain-grade is better than most other places excellent grade. My glass has been flawless.
MPEX is a strong second, but KEH tends to have better prices and a slightly better return policy.
My experience with Ritz has not been as good. Many of their older items seem to be graded based on their cosmetic/collectible condition and not necessarily their functional condition. I have returned three (out of four) cameras to them. One had a bad shutter and two had lenses with enough haze that it was apparent in the negatives. All three had been rated as excellent condition. The fourth, admittedly, was beautiful.
Depending on the make, swap meets can be great if you research exactly what you're looking for on the internet. For example, there were fairly comprehensive lists of what to check for on Rolleicords/flexes and Hasselblads in photo.net archives. Anything with electronic components might be a bit tougher. That said, I have easily gotten my best deals at the NJ Second Sunday camera show, and I wouldn't even classify it as a big show. Unlike KEH and MPEX, however, there's usually not a return policy.
B&H usually has nice stuff, but the prices don't reflect market conditions (at least not those of this decade). However, they often have hard to find bits and pieces. These don't show up on their website, but their magical back room has turned up answers to more of my odd requests (strange caps, oddball adapters, etc) than my bank account would like to admit.