Definitely both...but I'd give the edge to Negotiation.

With only so many opening moves, your negotiation skills will set your alliances in the beginning. You should be negotiating the entire game. I'm still befuddled when players do not speak in the beginning of a game. I almost always speak to everyone before the first turn, no matter what country I draw. It's always good to establish some rapport with a country that you may need help from later.

When negotiation has ended with a clearer definition of alliances and enemies, tactics are the skills you need to attack and defend. A good tactician can make quick gains and can fend off multiple attacks...which can lead right back to negotiation with your enemy.

Always look at the whole board to see what everyone is doing...who's allied with who. You can influence the opposite end of the board just by keeping the lines of communication open at all times.

It's definitely an asset to being a good tactician, but negotiation can always overcome tactics. Getting pummeled by your enemies? Talk to them! Talk to your enemies neighbors! No amount of tactics are going to save you if your enemies are convinced that you need to be eliminated.

I agree with Katsarephat. Depending on which country you draw, negotiation is invaluable in beginning of a game. But as the game progresses, tactics become ever more important, as your forces are growing to the point where allies are expendable and negotiations are trivial. You can certainly be a successful player with only one of these skills, but the better players wield both in their arsenal and can bring out whichever skills are needed in a given scenario.

That being said, I in no way consider myself in that group; I fancy myself a far better tactician than diplomat. However, that seems inconsistent with the country of choice: Germany. Germany, along with Austria and Russia, is one of the countries that must negotiate relentlessly from Spring 1901 through the mid game. I consider this to be one of my weak points, which is why I loathe drawing Austria, but, interestingly enough, I have won all three games on this site where I have drawn Germany.

Here's my take on xgongiveit2ya55's post:

Germany is a country with a number of tactical advantages. Germany usually has a lock on two, maybe three builds in the first year (something that cannot really be said about any other country without requiring a stab somewhere, except France). It can easily stalemate a French advance by land, it has a hefty buffer against a land-based Russian invasion from the east and a pretty good one against an invasion from Austria down south. It helps that neither Austria nor Russia can usually afford to tangle with Germany right at the beginning.

That said, most players (subconsciously at least) recognize all of this; for that reason, Germany has a lot of influence in any negotiation that goes on, and lots of people want Germany on their side (or at least not against them). Also, many people realize that good planning is needed to take a Germany down.

So as far as I can tell, the negotiation aspect of Germany is not intrinsic to the country at all, but rather follows necessarily from a strong tactical position.

I think the most important is something what is missing in your list -- strategy.

Tactics is trivial in Diplomacy, much easier than in Chess or Go, because there are not many combinations and looking ahead more then two turns is not necessary. So tactics is the basic skill that makes you just average player, not a good one.

Negotiation and psychology skills -- well, they are very important, but even if you master them, you will need a lot of luck to use them with reasonably good success ratio. They are more important and useful if you know your opponents well, but in the on-line games with people you don't know the potential of psychology and negotiation skills is limited.

Strategy is the most difficult to master, but it is the most important for success I think.

Tactics is not as easy as you think, MarekP. Just because there are not many combinations in Diplomacy, that doesn't make it necessarily easier. In Chess, only one player moves at a time. In Diplomacy, all players move at once, and must decide their moves, not knowing what their opponents are ordering simultaneously. This requires the use of guesswork or mixed strategies to be completely effective, and while still probably easier than Chess and definitely easier than Go, it does require some skills that are not present in either of those games.

Freaklag, you are right there are many possible moves played simultaneously in Diplomacy, but when you plan your orders, you needn't consider moves of other players. You have to consider resulting positions and the number of resulting positions is usually quite limited, especially if you eliminate nonsense ones first.

Of course, opening is an exception, but its theory is well documented and all average players know it.