What's the most recommended format should I use for fonts, OTF or TTF or another?
In looking for:
- Performance/Efficiency with SDL_ttf library
- Free/Open-ness 
 I'm planning to use GPL3 and Open Font License.
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If you want to use SDL_ttf, then you have no choice but using TTF fonts, see the doc:
Now if OTF is still an option you would consider (even if there's no SDL_otf, which means you'll have to implement it), it's potentially a better choice than TTF, for the reasons exposed here. It's also an open standard so that's a +1 for free/open-ness.
Still, fonts in game engines are usually simple bitmaps, for performance reasons. So if you're actually writing a game and performance is an issue, use an offline bitmap font generator as you were suggested in comments.