It basically means "so far", or "up until now":
This is the best iTunes so far.
I'd like to add that yet in the context of "the best X yet" is usually used when describing:
- something fairly new onto the scene,
- made by someone with an established track record of success,
- with hints that there may be more to come.
When describing something older, particularly in the twilight of a career or a product's life, you'd be more likely to use the word ever, instead of yet.
For example, I probably would not say:
Gimme Shelter is the best Rolling Stones song yet!
because that song was release in 1969. Instead, I would proclaim:
Gimme Shelter is the best Rolling Stones song ever!
Moreover, I'm not too sure I'd say:
When You've Got Trouble is the best Liz Longley song yet!
because Liz is an emerging artist without a large library of music to her credit. Instead, I'd probably be more likely to say:
When You've Got Trouble is my favorite Liz Longley song.
Lastly, I'm pretty sure I wouldn't say:
Heath Ledger as the Joker is his best performance yet.
because Mr. Ledger has passed away, and there won't be any future roles that may top his performance in The Dark Knight. I'd probably say instead:
Heath Ledger as the Joker was his best performance.
But, assuming I'm a fan of The Black Keys, I could see myself saying:
"El Camino" by The Black Keys is their best album yet!
because the album is fairly new, the group has put out a decent number of albums, and there is no sign of an immenent breakup of the band.