Dragonborn in particular is amazing. A whole new island to explore, with brand new assets for new dungeons and ruins, as well as new trees and flora to make it feel different enough from the base game, but still keeps that Skyrim aesthetic.
Dawnguard is good if you want a vampire character, or love hunting them, since it really expands on that gameplay and lore, and adds a new perk tree for vamp players. It's also very story-oriented if you're into that, plus it adds crossbows which feel pretty damn good. The story is still standard Bethesda quality though, so don't get too excited if that's a selling point for you =P
Hearthfire is the divisive one. I love it, but I also loved Settlements in Fallout 4. If you like the idea of having to build a home and customize it piece by piece, gathering or purchasing the necessary resources and materials (and even the land to build on) then you'll enjoy it. If you prefer just buying a house and instantly having it, then you'll probably find it a hindrance and ignore the new systems altogether. It only affects three new houses added by this DLC though, so Breezehome etc are exactly as they were in the vanilla game. Also, you can adopt kids, which may or may not be your cup of tea. But there are mods that add adorable Khajiit children so YAY!
So it's down to what you enjoy really. I'd say if you loved exploration from the base game then you'll probably consider the DLC worth it for Dragonborn alone.