After watching disjointed chunks of the second half of Jailbreak, I decided to check out the series. Watched Mirror Gem and Ocean Gem first since I really liked the melancholy around Lapis in the excerpts I've seen, and then a couple other random episodes, but it wasn't until I watched "Giant Woman" (as recommended by my brother) that it really clicked. I dunno why, but I thought Garnet giving Steven a rock and him honestly being excited about was great, and while a kid pestering his guardians to fuse all day could easily come off as obnoxious, Steven's "Giant Woman" song was too silly and cute for that (and a total earworm), plus he got me wanting to see Opal too, and the action sequence at the end was plain awesome. Then the conclusion with Garnet being once again too cool for school about everything and the "I can do that too" closing remark was a perfect ending. Since then I've watched a few more episodes, and am looking forward to more: they're my new entertainment during lunch breaks at work.
As for my favourite gem, so far it's Garnet, but I've only seen a few episodes, so it's too early to tell for sure: all the characters have their moments.