Most theoretical openings

What would you say are the most theoretical openings?

I define theoretical as having to memorize particular lines, especially forced lines, to avoid an unfavorable result.

The Spanish. It's been studied in tremendous depth since the 1500s and yet it is still rich enough for elite grandmasters to surprise one another in the first ten moves. 

The QGD Botvinnik is also extremely heavily studied and there are many positions that cannot be played without previously studying them.

The Sicilian Najdorf is another that requires a great deal of "book" knowledge to play well. 

But in every line, there is also room to investigate new ideas. 

Thanks for your responses so far.

How would you rank the following responses to 1d4 in terms of theory?

QGD-Orthodox

Slav

Semi-Slav (including the Botvinnik variation)

Grunfeld

KID

QID

Nimzo-Indian

Do you mean the QGD is the most theoretical or the least theoretical?

Semi-Slav Botvinnik
Sicilian Najdorf
Ruy Lopez Marshall
Grunfeld Exchange

I disagree. There may be a lot of theory on the QGD, but they're not the long forced lines where deviating means losing that he's talking about.

In fact those things are rare in general, I think that only really happens in the Botvinnik and the KID.

the entire list is somewhat dubious in my opinion.  

Here's what he said, in the original post:

I define theoretical as having to memorize particular lines, especially forced lines, to avoid an unfavorable result.

That's hardly the case in the QGD.

I don't know all these openings well but I think Fear Itself's post is closest to what the OP is asking for.

You can play the Ruy or QGD by strategical understanding without memorising lines.

It's easy to go wrong in the Semi-Slav Botvinnik if you aren't up on all the latest theory.  I think players who don't like to study theory are attracted to the QGD Tartakower because it seems pretty solid.  The Queen's Indian is another safe opening while the King's Indian Defence has its fair share of traps.  Theory in the Exchange Gruenfeld runs pretty deep.  The Nimzo-Indian isn't all that tactical, but White has a wide choice of responses, so you'd probably need to study a bit there.

As noted, theory has different meanings.  There is a large body of games played in the Queen's Gambit, but because of its solidity, I think you can survive to a playable middlegame without studying as much as the Botvinnik or Bayonet King's Indian for example.

riga i think Spasski had the same idea when picked that defense against Fischer ;)

btw what do you think about the queens gambit accepted??

I always think of theoretical as meaning 'sharp' - giving rise to lots of tactical lines -  on the edge of a bust.

I think of openings like King's Gambit, Pirc, KID, Nadjorf, non-Stonewall Dutch etc.

the grunfeld has the most theory: on wikipedia it says that the "latest" novelty occured on move 34 with Kd5! (black plays it) in the grunfeld.

heres the link: http://en.wikipedia.org/wiki/List_of_world_records_in_chess#Theoretical_novelties

Yeah, but all this is on the pro level... In REAL LIFE, the most theoretical are the Parham, the Philidor with 3...Bg4, the Damiano, etc.

I'd say the top 10 lines with the most memorizing to do just to survive are, in order starting with the highest:

1 - Botvinnik Semi-Slav

2 - Sicilian Dragon - Yugoslav Attack

3 - Sicilian Najdorf - 6.Bg5

4 - King's Indian - Bayonet Attack

5 - Ruy Lopez - Zaitsev Variation

6 - Semi-Slav - Anti-Moscow with 6.Bh4

7 - Morra Gambit - Chicago Defense

8 - Exchange Grunfeld

9 - Latvian Gambit

10 - Sicilian Scheveningen - Keres Attack

The QGA is probably not as theoretical as some of these others.  It's not as common either.

Add Caro-Kann, Panov-Botvinnik Attack to that list.