My friend just got back into Hearthstone for the first time in two years. His favorite class is the Shaman, and he plays only standard. I also stopped playing two years ago and have no idea what's going on in the game.
Which is the best expansion to gift him packs for the Shaman. I'd prefer to gift him packs from one of this year's expansions: Fractured in the Alterac Valley or Voyage to the Sunken City since they'll be in standard for longer; however, if one of the older expansions was great for the Shaman, that would be ok too.
I would say go for the standard bundle (1 legendary 10 packs) cause that's 2 guaranteed legendaries, and since he was gone for 2 years, remember that the new pity timer for unopened sets is 10 packs, so maybe try to mix them up (10 sunken city packs, 10 stormwind packs etc). That way he will at least have enough dust to make some decks. Maybe he'll even get lucky and get the shaman quest, which is kinda viable in wild right now.
Expansions rotate as yearly sets. So Voyage (as the expansion of the current year) would rotate in two years. All expansions of the last year (Barrens, Stormwind, AV) will rotate together next year.
On the matter of packs and returning players: there should be a free returning players deck (unless already claimed previously, I believe?), so go check them out if you are eligible. For packs, there is a guaranteed legendary in the first 10 of each expansion, which is probably better than 40 of a single expansion.
And duplicate protection, so you won't open the same card again until you have two of each in the rarity bracket, which helps you in completing common and rare cards rather quickly and ensures that you only open new legendaries (dusting does count as having had the card).
Voyage to the Sunken City pushes Multicaster Shaman, an archetype which revolves around casting Fire, Frost, and Nature spells. It also has a decent way to close out games with Bioluminescence on a wide board and a bunch of face spells. Then there's Piranha Shaman, which... exists, I guess. It's the first set of this year, so it's a good choice nonetheless.
Fractured in Alterac Valley is an amazing set for Shamans, both the Legendaries are good and there's a number of good-great Shaman cards there. Though it's only pushing one archetype: Freeze Shaman, and the returning player deck for Shaman gets basically everything of importance from this set, so if your friend got that, buying packs here won't give them much.
United in Stormwind is most notable for the Shaman Quest Command the Elements. It's not its time to shine, sadly, because Overload cards kinda suck right now, but you can try it if you want. Other than that, there's a Battlecry package which is really good and probably the real reason to look at this set, as Bolner Hammerbeak and Brilliant Macaw are amazing.
Forged in the Barrens' most useful addition for Shamans isn't even a Shaman card; it's Mutanus the Devourer, a card which synergizes amazingly with the Battlecry package from Stormwind, That's not to say the Shaman cards suck, but Elementals are kinda meh right now and the Murlocs are pretty bad.