I'd say it changes over time, first one brand then another, I'd also like to add that we've got a room with 16 pc's in it they've been running roughtly 12hours a day, 5days a week for 5 years now
two years ago we had 10 HDs die in a couple of months (IDE 40Gb Western Digital I cannot remember model)
I replaced with Seagates, as they were a slightly cheaper replacement cost. none of them has died yet. but at the same time the 8 remaining WesternDigital's are still going strong.