What are the best digital platforms for hosting online conferences that are more than just a bunch of panel discussions?

Two free solutions to check out for ZOOM events are:
BigStage.Online - a means to add 'call-to-action' buttons (such as Donate, Register, Volunteer) and feature sponsors in a panel that runs in a browser window next to your Zoom session. Presenters in the Zoom session can ask their viewers to click on one of the buttons which directs them to the URL you have set. This happens while they are still watching the presentation. The buttons go straight to your donation page so you get 100% of what was contributed. The buttons can be configured without programming and can also be used to conduct online surveys.
https://thedemlabs.org/2020/07/20/fundraise-better-on-zoom/
https://thedemlabs.org/2020/07/14/hiphop-politics-on-the-bigstage/

BigStageTeleprompter.online - A free on-screen teleprompter into which you post your script that scrolls by while you're presenting. This lets you maintain eye-contact while presenting on Zoom. 
https://thedemlabs.org/2020/12/21/stick-to-the-script-use-an-on-screen-teleprompter/
https://thedemlabs.org/2020/09/22/teleprompter-compatible-with-zoom/

Early in the pandemic a couple of organisations were really generous with their learning about how they've made conferences work online - you might find some helpful advice or tips in this article  https://www.niemanlab.org/2020/05/heres-exactly-how-we-organized-one-of-the-largest-virtual-u-s-journalism-events-to-date/ and https://digitalaction.co/news/lockdown-lessons

This article might be helpful for more social events (e.g. gathering after a conference): https://www.wired.com/story/zoom-parties-proximity-chat/

Halt the Harm Network and FracTracker Alliance recently hosted an environmental justice awards ceremony using Zoom and a social platform called Mighty Networks. They've used this platform to create The Campaign Network https://www.thecampaignnetwork.org to provide social platform for people fighting oil & gas. So far we've hosted a couple very interactive online events within the network. 

I also used the Mighty Networks platform to produce a couple of online conferences that had over 300 people logged in at the same time, with 5 simultaneous sessions. It was the platform's way of displaying events and RSVP that allowed us to have a conference fee.

I think this technology is still in its infancy and will rapidly evolve, but I've attended a couple of virtual conferences that used Hopin as the virtual platform. The features and drawbacks are very similar to the feedback about Whova, which I was not previously familiar with. I agree the networking and relational aspects of in-person conferences are very difficult to replicate online.

Hi, not quite answering your question but just sharing my perspective: What we tend to do is spice up the zoom sessions with additional digital platforms, like menti.com and https://mural.co/. In other words, we ask participants to log in to the additional digital platform while still being logged into the Zoom session, and use the secondary platform for the interactive dynamics that it offers. Also, we use break-out rooms often, sometimes just for 3 minutes to break the passive sit-and-listen dynamic.

I've seen two activist-ey conferences this year try to emulate some of the offline spirit in a covid-enforced remote switch, some interesting things: