I voted for..... No one!
Who has the best reliability seems to change from generation to generation.
6-8 years ago Western Digital was held in extremely high regard almost throughout the industry and had extremely low RMA rates. 3-5 years ago most people wouldnt dream of touching a Western Digital drive after their little fiasco with all the faulty chips they manufactured in one series of drives.
Within the last 2 years they've been slowly beginning to improve their reputation again.
Reliability varies far too much over time and between different segments of the market to pick any single drive manufacturer as 'the best'.
IMHO-Reliability:
The best mainstream 7200RPM IDE Drive: Maxtor's ball bearing D740X models.
Their FDB models see to fail noticeably more frequently.
The only viable competitors in terms of reliability seem to be Western Digital's BB line, and Seagate's Barracuda IV. I've not found the WD ''BB' line to be as consistent as the D740X BB models, and compatibility issues with older Abit mobos knock it from the top.
I don't consider Seagate comparable due to the Barracuda IV's extremely low reliability and performance when utilized in RAID arrays, granted you can RMA to Seagate for their Barracuda IV optimized specifically for RAID. Also too many of the initial shipments of the Barracuda IV suffered from extremely loud whining noises during seeks, this has since been entirely repaired but it removes Seagate from contention in my mind.
To briefly sum up the 'best' in the other markets IMHO:
5400RPM IDE drive: None. I do not believe any of the current generation 5400RPM models compare to select models of older generation drives from Seagate/Maxtor/Quantum.
10K SCSI: Quantum/Maxtor's Atlas 10k III. The Fujitsu MAN is also on top of the chain.
15K SCSI: Fujitsu MAM.
Seagate deserves honourable mention whenever SCSI comes into play as their SCSI drives are almost always extremely reliabile, and throughout their history Seagate has almost firmly dominated the SCSI reliability charts.