I picked Lightning Bolt as my favorite card (because it sees a lot more casual play than BSA!). I'm guessing 5 cards will have still been here forever, and that Giant Growth will stay, and Howling Mine will go. As a secondary, I'm guessing Bog Wraith will go too.
This will remain here until we know the true answers (apart from the "favorite card" which we'll never know), at which point I'll add an update for convenience's sake, showing how wildly inaccurate I was :)
-- Edit: The results are in! --
Giant Growth is indeed staying, and both Howling Mine and Bog Wraith are out. But having correctly called the "outs" is not all that impressive, as there are only TWO cards still in: GG and Giant Spider.