Newegg. I worked in a place that orders a lot of parts, we used many online shops but Newegg clearly takes the cake. For us it's the shipping. We can make an order any time day or night and know it'll get here withing a few business days. We count on it.
Other places you know, they'll say it's in stock but then we get a letter 2 days later saying it isn't. Or we get a wrong part, or it's not packaged right and the box is all banged up.
Returns are another issue, never had a problem returning at Newegg. I love the place.
I made an order from them on a Wednesday night around 10:30pm, the parts were at the Fedex office here on Saturday morning.
Once you know, you Newegg!