The most reliable are the Seagate SCSI drives properly cooled.
For IDE, Western Digital SE.
If you put a Seagate SCSI 15K drive and a Western Digital SE in the same system with out any cooling, the WD drive will probably last longer. If you add a hard drive cooler, the Seagate will most likely last longer.
For high speed drives proper cooling has a huge impact on reliability whether it be SCSI or IDE. I think more people cool SCSI 10K and 15K drives properly because it is absolutely necessary for them. Fewer people use drive coolers for IDE drives.
That isn't too say Seagate SCSI drives live forever. Last place I sysadmined enough data to get good numbers we had about 2TB worth of 2-18GB Seagate 7200 and 10K drives, the older drives tended to fail at around 5-6 years. Almost all drives were in constant use. It was very rare for a drive to fail before that age. It was very very rare for a drive to fail while running, it was almost always when an array was turned off and back on or when a drive was taken out and plugged in somewhere else that it failed. We tended to replace drives around the 5-6 mark where they were failing, some we beat some we didn't. I do not know how long the ones we beat would have otherwise lasted. This is out of memory, but I am pretty sure in the archive somewhere I give a better account at the point when I still had access to the reliability data.