There is a lot involved in getting a session just right, especially if you want it to be “ideal”. It is not possible to be perfect (or ideal) in every session. If we were then ideal would be normal and no longer ideal if you get my drift. It is possible to make an event memorable though. If you have the session where the players meet the dragon for the first time then there are ways to make that memorable. For me that is what an ideal session is – something that a player will remember for some time, if not forever. That golden haze around the “best game ever”.

Lead Up
There is more to building to an ideal session than just the preparation before hand. In all honesty the previous games need to be leading to a point where the player is anticipating the next game. They are bothering you in the week(s) lead up to it asking what is going to happen. Sure, you can run a session that does not need this and do it by surprise or even with a one-shot but if it is a campaign this can be one of your most powerful weapons. Layering tension on after other tension to build the drama before the resolution is a time honoured tradition of story telling so do not ignore the ability to ramp up your games in the preceding weeks.
I will continue on with the Dragon example but honestly the Dragon could be anything. A Dragon has a big effect on its environment and layered defenses. Even if you (in your preparation) roll a Dragon as the random encounter it is an event that needs some thought. A group should never walk into a cave and find a dragon there with no previous signs UNLESS something monumental has occurred and the GM has all of that covered as well! Dragon’s have agents and creatures that serve them. these servitors could be the lead in to the big event, coming thicker and faster as they near the centre of their search, leading the players to believe that something big is about to happen. This all takes place in the games that lead up to the “ideal” session though likely, in player memory, the story will grow to be larger than just the one session and factor these in.
Preparation
Knowing that the session is on you may require you to put in some more effort – it all depends in reality how prepared you are beforehand. I like to put in effort with a prop or two for an event I want to stick out. This way the player can take the memento and use it to elaborate their stories later. Also, making sure that you have the framework you need to create what you envision. I spend a lot of time in my games providing a sandbox for the players to play in and so in the leadup to a game I overindulge that sandbox. I make sure that things are perhaps a little more formed in the “ideal” game than they are in a regular game. This allows the players a much better picture of their world and can also bring out interactions that take you as the GM by surprise.
That is the style of game I run. I have a much better time in a game where the players surprise me than I do in a game where they do what I expect. That does not mean that this is the only way to play though! If you prefer to have the players move through the standard motions that is expected but still provide that ideal feel it is all good – just know a little bit of extra preparation can help in this area.
Comfort levels
This can really make a game. If you want the players completely zoned in on the storey maybe hold a pizza night before and have drinks and snacks available in a warm well lit room. Or if you want to run a horror session hold it in a barn on a wooden bench with a bit of a chill in the air – all of these things can make a difference.

Think of the effects you can get by varying the comfort level or location in a game. Trial a few games not in your regular spot, or just with candles at your regular table even. Go to the local library and have to speak in whispers or play on a picnic bench at the park – they all offer something to you as a GM, you just need to work out what that may be.
So, in total, for an ideal game that is the extra work I consider doing. I may not do all of it in the one session but you can pretty much guarantee that I have done some of it in one way or another. The “ideal” can be interpreted in a lot of ways but for me it is defined by the players remembering the game and talking about it for the years to come. I often fail to recreate games that occurs with but when it happens it is a magical thing! Keep rolling.
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It’s nice to see a long and detailed post for this question!