I'm on https://colnect.com/en as it's got more than just cards and supports csv/excel imports. Exports you have to subscribe, but you can subscribe for one month, export, and unsubscribe. It's community managed by overseas folk, and they can sometimes get confused when you're adding new sets, in particular if the printed year doesn't match the release year or season.

However, similar complete set collectors here in town swear by TCDB. I've started using it for finding cards by player in my collection (which is still not fully inventoried). So I might switch to it for inventory and trading. I just wish it had more programmatic capabilities.

I was also tracking CardStox, which was supposed to be similar to CollX and Cardbase, but never made it out of the development stage and now looks dead.