It depends on what exactly you want to get back to. If you are just looking for a strong deck, if you want to get your collection up to date, if you want to play mostly Wild or wish to switch to Standard.
Since you are mainly asking about sets, I'll try to break down what you've been missing and what you might want to get for whatever purpose:
The Witchwood: A set with a relatively low impact and few good cards in it. 3-4 legendaries are still good, and each class has maybe two or three decent cards that are still useful. The neutral section isn't looking much better. You want mostly commons and a few rares and a handful of epics, that are still good/interesting. Unfortunately, the undeniably best cards of the set, Genn Greymane and Baku the Mooneater along with a few cards that worked exclusively with them, were prematurely rotated to the Hall of Fame in April. In other words, you can no longer get them in packs, and would have to craft them. These two legendaries had a huge impact (which is why they were rotated) and are still quite relevant in Wild (Baku works extremely well with Paladin, Rogue and somewhat with Mage and Warrior, Genn is great for Shaman, Warlock and is also ok for Paladin). Generally, Witchwood isn't too good, and you might just want to craft the cards you want from the set.
The Boomsday Project: A set that initially did not take off, but is now one of the most important sets in Standard. It has some decent cards for Wild as well, but isn't that important. It holds several strong cards especially for Druid, Hunter, Mage, Paladin and Warrior. The neutrals also have some fairly good cards. Especially Zilliax is very solid across many different classes and a card likely worth getting. Certainly a set I'd purchase a few packs from.
Rastakhan's Rumble: Another set that has a handful of decent cards, but most of them are not that great. Similarly to Witchwood, you want to get maybe two or three cards per class, and a couple neutral cards and that's about it.
Rise of Shadows: Pretty strong set with a high impact for Standard, and some for Wild too. Has some very good cards for Druid, Mage, Priest, Rogue, Shaman and Warrior, and some very good neutrals. Obviously worth getting lots of packs from, if you want to focus more on Standard since it won't rotate for quite a while, and still worth some investment if you care about Wild. As for the Dalaran Heist: It is primarily single-player content and beating wings will just award you with packs (3 per wing). Getting and beating the entire adventure awards you with card backs, a golden Classic pack and a golden Zayle, Shadow Cloak, an unimpressive legendary. Unless you care about single-player stuff, it isn't really worth it.
Saviors of Uldum: It's a bit early to say how big its impact will ultimately be, but it already looks pretty decent. Zephrys the Great is so strong that it inspires Highlander decks in Standard again, and needless to say it is a very good card for Wild Reno Decks. But it came out just a week ago, and it's still a bit early to tell. Since you should be getting some promotional stuff for it still, just grab what freebies are handed out and maybe get a few extra packs from it.
Bottomline: Boomsday and Rise of Shadows are pretty good sets, Uldum looks good for now, while Witchwood and Rastakhan are not really worth buying too many packs from, but hold a few decent cards worth crafting.
Current meta in Standard: Control Warrior is an extremely strong deck, and not unlikely to get nerfed, since it is virtually the same since the launch of Rise of Shadows, and few decks can reliably beat it. Hunter is looking pretty good with its Mech, Secret and Highlander builds, and Mage has some solid options too. The rest of the classes are still in an experimental phase, and the next two weeks will show how the meta eventually turns out.
Current meta in Wild: Big Priest continues to be a hot topic (almost the same deck you might remember from Kobolds and Catacombs) that many people would like to see go away. The rest of the meta is fairly aggressive, mostly just to beat Priest: Odd Paladin, Odd Rogue, Mech Hunter, Murloc Shaman and Even Shaman are all decks that try to end games quickly while having a solid chance against each other. Quest Mage (with Open the Waygate) is also a very strong deck that is a bit more combo-oriented in playstyle.
Feel free to ask if you have more specific questions about decks or cards or whatever else might concern you.
It depends on what exactly you want to get back to. If you are just looking for a strong deck, if you want to get your collection up to date, if you want to play mostly Wild or wish to switch to Standard.
Since you are mainly asking about sets, I'll try to break down what you've been missing and what you might want to get for whatever purpose:
The Witchwood:
A set with a relatively low impact and few good cards in it. 3-4 legendaries are still good, and each class has maybe two or three decent cards that are still useful. The neutral section isn't looking much better. You want mostly commons and a few rares and a handful of epics, that are still good/interesting. Unfortunately, the undeniably best cards of the set, Genn Greymane and Baku the Mooneater along with a few cards that worked exclusively with them, were prematurely rotated to the Hall of Fame in April. In other words, you can no longer get them in packs, and would have to craft them. These two legendaries had a huge impact (which is why they were rotated) and are still quite relevant in Wild (Baku works extremely well with Paladin, Rogue and somewhat with Mage and Warrior, Genn is great for Shaman, Warlock and is also ok for Paladin). Generally, Witchwood isn't too good, and you might just want to craft the cards you want from the set.
The Boomsday Project:
A set that initially did not take off, but is now one of the most important sets in Standard. It has some decent cards for Wild as well, but isn't that important. It holds several strong cards especially for Druid, Hunter, Mage, Paladin and Warrior. The neutrals also have some fairly good cards. Especially Zilliax is very solid across many different classes and a card likely worth getting. Certainly a set I'd purchase a few packs from.
Rastakhan's Rumble:
Another set that has a handful of decent cards, but most of them are not that great. Similarly to Witchwood, you want to get maybe two or three cards per class, and a couple neutral cards and that's about it.
Rise of Shadows:
Pretty strong set with a high impact for Standard, and some for Wild too. Has some very good cards for Druid, Mage, Priest, Rogue, Shaman and Warrior, and some very good neutrals. Obviously worth getting lots of packs from, if you want to focus more on Standard since it won't rotate for quite a while, and still worth some investment if you care about Wild.
As for the Dalaran Heist: It is primarily single-player content and beating wings will just award you with packs (3 per wing). Getting and beating the entire adventure awards you with card backs, a golden Classic pack and a golden Zayle, Shadow Cloak, an unimpressive legendary. Unless you care about single-player stuff, it isn't really worth it.
Saviors of Uldum:
It's a bit early to say how big its impact will ultimately be, but it already looks pretty decent. Zephrys the Great is so strong that it inspires Highlander decks in Standard again, and needless to say it is a very good card for Wild Reno Decks. But it came out just a week ago, and it's still a bit early to tell. Since you should be getting some promotional stuff for it still, just grab what freebies are handed out and maybe get a few extra packs from it.
Bottomline: Boomsday and Rise of Shadows are pretty good sets, Uldum looks good for now, while Witchwood and Rastakhan are not really worth buying too many packs from, but hold a few decent cards worth crafting.
Current meta in Standard: Control Warrior is an extremely strong deck, and not unlikely to get nerfed, since it is virtually the same since the launch of Rise of Shadows, and few decks can reliably beat it. Hunter is looking pretty good with its Mech, Secret and Highlander builds, and Mage has some solid options too. The rest of the classes are still in an experimental phase, and the next two weeks will show how the meta eventually turns out.
Current meta in Wild: Big Priest continues to be a hot topic (almost the same deck you might remember from Kobolds and Catacombs) that many people would like to see go away. The rest of the meta is fairly aggressive, mostly just to beat Priest: Odd Paladin, Odd Rogue, Mech Hunter, Murloc Shaman and Even Shaman are all decks that try to end games quickly while having a solid chance against each other. Quest Mage (with Open the Waygate) is also a very strong deck that is a bit more combo-oriented in playstyle.
Feel free to ask if you have more specific questions about decks or cards or whatever else might concern you.