Filmfreeway, or the festival's own site. I put up with Withoutabox's crap for years, and heard from organizers about what bullies they were, so I was only too happy to jump ship. I only use Withoutabox if there is no other submission option.
I'm old enough to remember the pre-Withoutabox days, so I have to give them some credit for making what used to be an even more exhausting process more bearable. But I find Filmfreeway really nailed the UI, their customer support is better, and they took the time to listen to filmmakers to find out what we need to see onscreen and how we think