It's almost impossible for anyone to make an informed recommendation without a lot more information about your needs. There is no perfect solution, because everyone's needs are different. There's only a perfect solution for you.
Do you need inventory management? If so, do you need inventory tracking on product options (like colors and sizes)? That will really limit your options, because a lot of the open source and inexpensive shopping carts that you can just install on your site don't have option-level inventory tracking.
What about shipping and fulfillment? Will you need real-time integration with UPS, FedEx, USPS or just flat rate shipping based on order total/weight? Or will you be drop-shipping?
Are you selling physical or downloadable products? Will you need to calculate sales tax? Do you need merchandising, coupon codes, etc?
If you decide to go with WordPress, WP E-commerce is one of the most popular plugins. It's nowhere near as full-featured as ANY actual shopping cart or hosted e-commerce solution like Shopify or Bigcommerce, though.