Competitor on what vector? Speaking from a US-centric viewpoint here, but my thoughts;
* Distribution & Warehousing - Walmart & Costco
* Sales & Advertising - Google & Facebook
A few notable online storefronts that are independent and I use frequently are B&H Photovideo and Newegg. Realistically though the options I listed above are the only companies I see having the scale to compete with Amazon at anything, and even then they're an order of magnitude behind. Just my opinion, again very US-centric.