Reg, just jumping in until Paul arrives. Flac is indeed a larger file than AAC. But for the sake of brevity and not re-opening the old 'mine's bigger than yours' argument, it's entirely your choice. There are a few different option available but two things to consider: one, your Internet connection. Is it consistent? Or is it dodgy? Bigger files (Flac) will take longer to download than AAC, MP3 and the usual. And two, what format is most compatible with your computer? For example, I cannot play Flac files just by clicking on them. I need to download third-party options just to play them. Then if I want to burn Flac files to disc, then I would need to download another bit of gear to do that.
So once that's addressed then you have to consider sound quality, i.e., can you hear the difference between any of the files on offer? If you think you can and insist on downloading the biggest file type, then let that be your deciding factor. If you are still unsure about any of this then you'll need to ask some more questions. Hope that gets the very basics covered without confusing matters.
James, it's a monster from the 'Great Book Of Senseless Mythologies'! But seriously, it's one of the oldest and most obscure audio file types meant to sound impressive but the last known user died in 1904, uttering in his native tongue of Manx, 'You know, I really can't be bothered hanging around here waiting for the invention of the personal computer', and dropped dead on the spot.

But best is a wholly relative term. What's best, a Tesla, or a sound working, affordable car? Sometimes we are spoilt by the illusion of choice.

Mind you, Paul might have a much more helpful answer.