How can I stream (Jackbox) with the lowest latency possible?

Unless you're planning on allowing people other than your group of friends to watch the game, any group video chat or screen sharing software would be perfect for your needs.

Google Hangouts have incredibly low latency (< 1 second in my experience) and allow you to share your screen with others in the chat. Google Hangouts are also free if you already have a Google account. I've personally used Google Hangouts for this purpose, and many other times I've had to share my screen / look at someone else's screen. While there are some quirks if you also speak through the Hangout, this shouldn't be a problem for you if you're using Teamspeak.

As another option, join.me is a screen-sharing alternative that is free, lightweight, and easy to configure.

Just for 2021 and onwards a new view on this topic:

While Twitch and Youtube (and Steam Broadcast) still focus on massive amount of viewers (obviously), latency still takes a hit with that services. Apparently there are ultra-low-latency modes in Youtube and Twitch (look up this link) and they are "fine" but it's still meant for streaming for a big crowd, so it's still not the best way do play Jackbox Games.

Discord supports screen sharing in private calls and in voice chats on servers. These are still limited to few people (compared to Youtube and Twitch) but enough for any Jackbox Games round. As they are so relatively limited in terms of viewer count, it allows the stream to have way less latency. This is probably the easiest and best way for anyone to play Jackbox Games over the internet.

In contrast to Steam Broadcast, the 2019 released service Steam Remote Play streams not only the screen from the host to the viewers but also button inputs from the viewers to the host, as it is designed for any local coop game to work over the internet. The input transfer is not important for Jackbox directly as the interaction from the viewers works over jackbox.tv anyway, this fact does show that this service is completely focused on ultra-low latency screen sharing because that's what's needed for playing local coop games online, even more than for Jackbox Games. This puts Steam Remote Play maybe on an even better spot than Discord in this field. Nowadays it can even be shared through only a single link, meaning not even a Steam installation (and afaik not even a Steam account) is needed at the viewers/clients end.

Google Hangout apparently also has very low latency but I haven't tried that yet.

For what it's worth, my personal experience was that Hitbox has far less stream delay than Twitch, which was why I preferred it over the more popular site. Something closer to 5-7 seconds was the norm, from what I recall. I haven't personally tried Steam Broadcasting in any significant amount, so I'm not sure how it compares.

(Just in case, disclaimer: I have no affiliation with any streaming service, other than using Hitbox on the rare occasions that I stream.)