ESO is still getting new content four times a year, so there's no definitive way to get everything (for now).
Content in ESO is split into two categories:
Chapters are big expansions once a year (second quarter) that bring one big zone, a new story line, and some other big features and/or a new class. So far there've been three chapter releases:
- The Elder Scrolls Online: Morrowind including the Island of Vvardenfell, home of the Dunmer, which was also featured in TES3: Morrowind. This expansion introduced the Warden class (similar to a classic druid).
- The Elder Scrolls Online: Summerset including the Island of Summerset, home of the Altmer and Artaeum. This expansion added a new skill line (Psijic Order) and a new crafting discipline (Jewelry Crafting).
- The Elder Scrolls Online: Elsweyr including the northern part of Elsweyr, home of the Khajiit, as well as the Necromancer as a new playable class.
DLCs, which are smaller content releases typically falling into one of two categories:
- Story DLCs introduce new zones (smaller than chapters) with new stories and sometimes new features. There's typically one new story DLC a year right before Christmas (4th quarter).
- Dungeon DLCs each include two new instanced dungeons for one group of 4 players (with a new story, items, etc.). These typically release in first and second quarter of a year.
If you're absolutely sold on getting the game, get The Elder Scrolls Online: Elsweyr, as the latest chapter (purchased as a full version) always includes all previous chapters released as well as the base game (this doesn't apply to the "Upgrade" versions, unless it's a pre-order of the upcoming chapter).
If you're undecided, just look around. There are different editions (as you've noticed), most starting at around 5-10 bucks (if on sale or sometimes even retail). As you've noticed, some include additional DLCs, but I wouldn't necessarily focus too much on that.
This won't get you all additional DLC, however some DLC is given away for free now and then, plus there's the game's optional subscription (ESO Plus), which gives you unlimited content to all DLCs as long as your subscription is active/paid (in addition to some other perks like more storage space, more room in your houses, etc.).
Do you need the subscription? Absolutely not. You can grab DLCs one by one for a fixed price (or sometimes in bundles or deals), but you can also subscribe now and then. Subscribing costs around 15 bucks a month, but will also get you the Crowns (premium currency) you'd get for the same amount of money. So you can sub a month or two, try all the DLCs for "free", and then cancel the sub and buy the DLC you enjoyed the most using the Crowns you got.
Considering the amount of content in the game and the DLCs (I'd say this dwarfs Skyrim by a significant amount), you're perfectly fine going with the base game in the form of Elsweyr for now (at least as of the time of this writing):
The base game alone includes around 100-150 hours of questing per alliance (three total), additional shared areas and guild content (probably another 50-100 hours), and the chapters with another 30-50 hours each. Specific numbers are hard to estimate as everyone plays differently. If you're listening to all NPCs, reading books, looking for things, etc. you'll definitely reach those numbers. If you rush through, then you can probably cut most of these by half or more.