Globo Product Options, Variant

This app is really easy to customize, and the developers help out quickly if you get stuck anywhere. A very powerful feature set especially useful if you want to display custom text boxes, upload files. It also has very useful conditional logic to display or hide options.

Edit: Review after using for a month - definitely found an increase in upsells after using this app! The UI and integration with product page is very seamless, ever offers easier and better options than the native Shopify theme.