The Short Version: I've been building my own computers now for about 10 years and I've always stuck with Western Digital. They've always been reliable for me, but Seagate looks promising too.
The Long Version: I have a 20.5GB WD that's about 8 years old and still chugging, but I worry about it. Every now and then there is a mysterious click. I also have a 40GB WD, a 100GB WD, a 160 GB WD, and then I got a 200GB Seagate. Most of the computer builders I've known through the years strongly recommend WD. Some say Seagate is crap, others love seagate. I've had no problems whatsoever with my seagate and it's a lot more quiet than my WD drives, so I love it. I was having some problems with my 100gb WD drive, so I replaced it with a 160gb drive, but then it turned out that it was my motherboard going bad causing the errors. After replacing the motherboard and a format, the drive was fine. Oh, the 200gb Seagate is about 4 years old.
I've known a few people who bought cheaper IBM drives and they had problems. Samsung seems like a new contender, but from the reviews that I've seen, I'd stay far away. They seem to have a lot of problems. I'm sure you might find a diamond in the rough with certain models, so I suggest you go by reviews when buying a hard drive and only use brand name as a general indicator of quality. A certain model of a WD drive might have a lot of problems too, you know.
Well, good luck. :)
Phil
P.S. Oh, one more suggestion. If you want a more secure computer, I suggest you get two smaller, cheaper hard drives for your operating system and for some storage space for important files. Put it in a RAID 0 configuration, then get a larger hard drive for putting everything else on. This way if your hard drive goes bad, it's no big deal. If you don't want to spend extra money, I still recommend that you buy 2 hard drives rather than 1 super large one. It's quite a hit if a 500gb hard drive with everything on it goes bad you know.