It's a pretty common belief that having the best tank (or fighter jet, or light machine gun, or whatever other piece of "cool" equipment) will matter a lot when it comes to who wins a war. In popular military history, you see a lot of this - T-34 vs Tiger I, F-15 vs Su-27, which one is better? It's relatively easy for the layperson to understand and there is a lot of popular interest in certain types of military weapon systems - tanks and fighter jets in particular.

In reality, these differences really don't matter much on a strategic level. Wars are very complex and messy affairs and there are a lot of factors involved that have nothing to do with the quality of the equipment. Things such as doctrine, average training levels, logistics, terrain, intelligence and coordination between different military branches all matter a lot more than having a tank that is slightly better than the enemy's tank. It doesn't matter if you have the best tank in the world if it isn't in the right place at the right time, and things like reliability and strategic mobility help a lot with that. A lot of military thinking centers around gaining local superiority - an advantage at the right place at the right time.

On a tactical level, though, in tank duels between tanks of roughly the same generation (in other words, tanks that both have a reasonable chance of damaging each other), by far the most important factor is shooting first and hitting first. Even if you miss, you will be able to correct your fire and get a second round off faster, and even if you don't damage the other tank in any meaningful way, being under fire has a strong psychological effect. A Swedish Army field manual on tank gunnery basics from 1974 went as far as to claim that the tank that fires first will destroy the opponent four times out of five, and the tank that hits first is four times more likely to win than the tank that doesn't. These claims were probably based on statistics from WW2, but the general idea that getting the first hit in is a big advantage in tank vs tank combat persists even today.

With all of this said: I'm not familiar enough with the Six-Day War or the other Israeli conflicts to make an informed answer that covers those, but in the Gulf War the odds stacked against the Iraqis were just impossible to overcome, both strategically and tactically. The coalition had total air superiority, vastly superior intelligence due to satellite coverage and things like the JSTARS aircraft, the American tanks and IFV's had observation and night vision equipment that provided an enormous advantage over the T-72's infrared searchlight (remember, shooting first is a big advantage, and seeing the enemy tank first is an important compoenent of that), and on top of all this the Iraqi T-72's were using a relatively old type of armor-piercing ammunition that had a very hard time penetrating an Abrams tank from the front. There were more factors (logistics, morale, operational planning...) but really it's enough to say that the coalition had very big advantages in almost every conceivable area.