The palantiri= one of the main reason for Gondor's success against Mordor?
 

Gondor use of the palantiri for spying on the enemy and launch preemptive strikes against Sauron's forces, was one of the main reason why they could withstand him for so long?

Don't think so. I doubt many Stewards used the Palantir, or used it often. Denethor is supposed to be the exception. And with such frequent use, Denethor's perception was warped by Sauron, distorting the visions to show despair rather than hopeful strategy.

The one strike we see Gondor launch is Faramir's attack on the Haradrim unit marching down the road. The reason they were able to set up the successful ambush is because of good Ranger scouting, not because of information from the Palantir.

In other words, Gondor has a good military which is not dependent on the presence of the Palantir. And the presence of the Palantir turned into a weakness rather than a strength in the end.

The palantiri weren't useful as devices from a military operations standpoint. Unfinished Tales: The Palantiri is probably the most detailed and comprehensive information Tolkien wrote on the topic so I won't quote the chapter in full...but some of the main points.

They were presumably made by Feanor and their total number is unknown. But 7 were brought to Middle-earth by Elendil, Isildur and Anarion to be ways of communicating between their 2 founded Kingdoms, Arnor and Gondor.

The heirs of Elendil (and eventually the Stewards of Gondor) were said to be the 'rightful possessors' of the stones brought to Middle-earth, which is important in their use. This means the vast majority of people would have had no use for the seeing stones. If they were even interested in them anyway, as their perception was described as rather 'alien' to Middle-earth and devices only Elendil and his rightful heirs would have contemplated using.

There was a 'general strain' when using the stone, even for the rightful heirs. The viewer would have to concentrate a considerable amount of will to direct the visions/images they wanted to see, otherwise Tolkien describes the viewer would just see a bunch of random and unconnected images. This strain was considerable and noted why Denethor appeared older than he actually was.

Sauron only used the stone he captured when taking Minas Ithil as a minor part of his plan to weaken Gondor's resistance from within and attempt to control Saruman.

Overall their benefits were quite limited, yes and others could gain great knowledge but at a tremendous and dangerous cost. There were a limited number of people who could make use of them and they were still dangerous to those people who could. One of Sauron's biggest blunders was made because of the palantir when Aragorn used the Orthanc stone.

What about during Arnor's battle with Angmar? Wouldn't the stone be useful to expose the Witch King's forces whereabouts? Or Numenors war against Sauron?

My understanding is that the Weathertop stone was indeed used as a tool for spying - but mostly on other Arnorians. Cardolan and Rhudaur were apparently always fighting over it, since it was the only palantir either of them had a chance of claiming. It's hard to see that as being because they just really wanted to chat with Minas Anor; they must have had a practical purpose. (Weathertop was also the 'master-stone' of the North, which presumably afforded it some control over the Annuminas-stone; again, as a tactical matter, claiming the former to disable the latter implies some military use.)

Unfinished Tales essentially describes them as magical telescopes: an untrained/unauthorised user would see things essentially at random, but if you knew how to point it, you could focus on a single point. The full description is quite fascinating; I recommend it if you're interested in the Stones.

Would they also have been useful against Angmar? I'm not so sure. Unfinished Tales says:

Would the Witch-King be able to shroud his armies and render them invisible? It's hard to imagine he wouldn't, given that he must have known the Stones were in play. Perhaps even more significantly, the Stones were designed to focus on a point, not a person or threat. You could use them to confirm an army was present where it was supposed to be, but not to hunt for one without meticulously and exhaustingly trawling the countryside.

I'm sure Pharazon would have loved that, but the palantiri were given to Amandil (father of Elendil), leader of the Faithful. There's no way he would have told the king about them.

hS

Exactly. It's like hunting for enemy warships using spy satellites: if you don't already have a pretty good idea where they are, good luck. The ocean is a big place.