Scrabble app for my iPhone. It's also available for the iPad, should you have one of those.
If I could describe this app with one word, I don't know if I would choose "addictive" or "excellent". How about addictively excellent? Leslie and I had been playing Words with Friends (another great app and one we're still playing) and now we typically have a Scrabble game constantly on-the-go as well.
You do have to like words to like Scrabble; if spelling and wordplay are not your thing, then you likely wouldn't enjoy Scrabble at all. I'm a word junkie, so Scrabble is most definitely my thing.