NOTE: This platform is currently banning all NSFW content. While it definitely is a good host when it comes to permitting political speech of all stripes, this technically means it isn't truly a free-speech platform the way the other options are. I may be removing entirely in the near future
Gab is probably, on its face, the most famous free-speech platforms out there. The creator, Andrew Torba, created the service after he saw that Twitter was moving in a direction that censored people that the company Twitter didn't particularly agree with and also censoring people to protect their brand from advertiser backlash.
Having said that, the major draw back is their reputation as a retreat for some less-savory characters that have been banned from Twitter for legitimately inappropriate behavior, including harassment of other users and bigoted content, primarily centered around anti-semitism. As a result, the stability of the platform has been threatened in the past, like with their website domain being cancelled, their payment services being revoked, their azure infrastructure being threatened, and their apps being removed from the AppStore and Google Play, and some people might be turned off by the reputation and the presence of bad actors on the platform. I would say proceed at your own risk when using this service.
Edited: 7/8/2019: Gab is now a heavily customized fork of the Mastodon software noted above. This means that it is open source and accessible by apps like Fedilab and Tusky Uncensored. Some apps have chosen to block the Gab Social instance but many still work. Similarly, many large Mastodon hubs like Mastodon.social and others have chosen to block the Gab Social instance as well to prevent users from reaching the content their administrators find objectionable or (in the case of mastodon.social) illegal in their communities.