For this volume we get the Brother HL-L6200DWT. We have primarily Brother printers and have been very satisfied. Printer and ink costs seem to be among the lowest we've found.
Agree that the HL-6200 model is solid
The best were the HL-5370DW. We just had two that FINALLY gave out around 150,000 pages. We still have about 5 or 6 around and they simply work. Never have issues just replace toner and the occasional drum.
The HL-5400 series had issues with their fusers, mainly if you were a decently high volume printer (1000+ pages per month). Less than 500/month we haven't had the same issues.
We have two HL-L6200 currently. The first one we bought kept creating creases down the middle of the page. Apparently it's an issue others have experienced as well and Brother exchanged it. The new one hasn't had that issue yet and it's been in place over a year now.
The only complaint I have is how the toner acts when it gets low. Many times when the printer says it's time to replace toner, the quality is still fine so you can reset the counter and keep printing. Unfortunately if you do that, eventually you'll start seeing toner slowly seap out onto the pages and if users aren't aware of what it is and let it continue, it can create a mess. We just train our users to change toner out at the first sign of this.