I like Best Buy - they have a large selection and most of the time, the employees actively offer help. I'm not sure that all of them are knowledgeable about what they're selling, so do your homework before you go.
They are certainly not the cheapest electronics/computer store but offers the convenience of one-stop shopping and you can feel pretty confident that they'll have what you are looking for. There are times, however, when they offer fabulous deals (e.g. during the holidays).
They also offer a pretty nice rewards program based on purchases. Most employees are very pleasant and helpful (i.e. offer to help load heavy items into your car). They've also paired up with GeekSquad to service your computer on-site or at your home.
PROS: Large selection of electronics and computers, one-stop shopping
CONS: Not lowest priced, not all employees are well-educated on product
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