A fiew opening questions I have

Queen's gambit is better

Open sicilian is best for white

Stonewall, Leningrad

semi-slav is good

Openings are largely a matter of taste.  Here are some tips for when you learn a new one:

http://www.associatedcontent.com/article/1671139/chess_101_learning_a_new_opening.html?cat=9

He's asking about the danish gambit, not the dutch.

The queen's gambit is more mainstream but they are both played at the highest levels of chess.  It has more to do with the types of games you enjoy playing.

The KG isn't really played at "the highest levels" of chess with any regularity whatsoever.

fisher ans spassky played it and they are "world champions"

Fischer rarely played it, Spassky didn't use it as his main weapon, and neither of them has been playing at the highest levels in 30 years

Note the caveat "with any regularity" ;he is conceding that it does in fact get played at the highest levels, just not that often.  It's a controversial opening, which you can see by looking at all the assorted king's gambit threads that have sprung up.  

The Grand Prix Attack (1e4,c5 2f4...) is a good line to use versus the Sicilian Defense if you like to attack. The King's Gambit is a good weapon (as White) against a club player...

Currently the KG isnt popular at the elite level and in my life it never has been.

Here's the thing with openings: If you and your opponent are not playing at master level, almost anything goes. Even at competent club level you can (and should, because it's fun) play things like the Blackmar-Diemer, or King's gambit, or even highly speculative things like the Halloween or Hammpe-Allgaier gambit and get away with it.

And even if you are only an average player, and your opponent is very strong, DONT play popular book lines.

Think about it this way, you have to play against Kasparov. As a 1600 you probably have 1 in 100000 chance of taking a half point, and he will have to make a mistake(which you will have to identify and exploit). Ask yourself, will he more likely make the mistake in a main line Ruy Lopez, which he has played a gazillion times, or in a position he has maybe not seen before?

I know I am almost alone in this line of reasoning, but I don't care, so there!

I don't think you're alone! Objectively, as long as it's not the Wandering King Attack any opening if learned thoroughly can be fine at most levels.

No one said it was in the first place.  

Kasparov hasn't seen a position before? You're kidding right?

I'd definitely recommend book lines against stronger players, since they are more likely to know the refutations to non-book lines. If a stronger opponent doesn't know your ability they are more likely to agree to short draws or other concessions if you play a strong opening than any old rubbish.

 Think about it this way: if you follow high level theory against Kasparov, you are playing the moves that centuries of theoretical development has deemed best, and your weakness doesn't mean a thing; Kasparov will have to go into something inferior to let you think for yourself.

Go into something unknown, it's a 1600 playing Kasparov.

...and the Queen's Gambit isn't really a gambit at all!

1. I prefer the King's Gambit myself. The QG gets you into a lot of Slav/Semi Slav theory.

2. I like the Closed Sicilian. And as a bonus, it doubles as a defense for black against the English.

Or, think about it this way, lol. For me it works because:

I have always a fast player, (after 1000 and plenty games here my time per move is 20 minutes) OTB is the same, I don't recall ever being in time trouble. I often find this induces opponents to move faster than their own comfort level, even strong players, I don't know why, it just is...

Playing like this, I often blunder, and lose to 1200's (I'm stable around 1600). But, my opponents also often blunder, even the strong ones. And probably always in positions they don't know. I frequently beat or draw 1800's, I have beaten a handful of over 2000, and have a draw against a 2300. Far more good results against stronger players than my rating should allow. My games are filled with Vienna Gambits, Nimzovitsch defenses, Budapest Gambits, Polish openings, and a few 1e4 b5?! as Black.

So, maybe not everybody's cup of tea, but I'm having fun....

1. The Queen's Gambit has many of the advantages of the King's Gambit in challenging for control of the center, without the risk--the pawn is easily regained (which is one reason the gambit is usually declined), and the kingside is not opened up to possible counterattack. For this reason the Queen's Gambit has historically been considered the better.

2. Because so many lines of the Sicilian are known and played and are so complicated, I prefer as White to evade all the common lines by playing the Closed Sicilian Defense.

4. Most people who don't want to face a Queen's Gambit cultivate one of the Indian Defenses, such as King's Indian or Nimzo-Indian. I prefer the Benoni or the Leningrad Dutch Defense.

There are two different approaches being discussed, hence the disagreement.

1. Pick an opening that is unusual and has little theory.

2. Play non-book moves in an opening that has a lot of theory.

With the first approach, you can study the unusual opening and be better prepared than a stronger opponent. The second approach is probably a bad idea for club level players.