I used to go to Diapositive and then LTI when Dia closed. Also used coloredge, 68 degrees, etc. But as of last year I started sending my film to Praus in Rochester...definitely superior to anything in nyc. The difference is that Edgar runs a one man operation and he's a photographer to boot. Eastman Kodak routinely sends him materials to test as his processing is as clean as it gets. At the end of the day the cost of producing my work and the effort I put into it warrants the extra time it takes to send to Rochester.
I've noticed a distinct fall off in quality at the prolabs in nyc in the past several years. It got to the point where I could no longer trust any lab in the city to do consistent work. A lot of labs have high employee turnover, operators who are wage slaves, oftentimes migrant workers, just going through the motions with no real eye towards quality or cleanliness. And the celebrity labs like MV pimp their reputation and charge far too much. The person at the front end can talk the talk and con you into giving them their business but if you could see what happens behind the curtain you'd gasp. I've seen laughable work come out of MV.