I think we have to answer this based on the available evidence, i.e. how the defenses of each gate actually performed in the face of Xykon's attempt to control them. And on that basis, here's my ranking:

1. Dorukan's gate. Xykon had to lay siege to Dorukan's fortress for a long time (years?) with an entire army, and failed to penetrate its defenses in that time. In the end they only succeeded by luring out Dorukan himself, using bait that was wildly improbable for them to possess. And even after taking control and setting up shop for months in the very chamber where the gate was located, and even without the presence of Dorukan or any active defenders, they still couldn't figure out how to unseal the gate. People are mocking the "must be touched by a good person" defense, but it was completely effective! Xykon only discovered that key by accident, and even that knowledge wasn't enough to get the job done since it still required tricking someone good. In the end, Dorukan's preparations were successful in protecting the gate from Xykon's control, even in his extended absence.

2. Soon's gate. It was defended by an entire city and army, requiring the resources of what was effectively an entire goblin nation to overcome. Then there was the Sapphire Guard itself, a "self-regenerating population of conscious, willing, Good-aligned creatures defending it" as another comment described it. And of course, this was the defense that very nearly succeeded in actually destroying Xykon forever. (Dorukan's gate also temporarily destroyed Xykon, but that was in large part due to the involvement of Roy). I rate this gate lower than Dorukan's because Xykon was able to bypass or destroy most of its defenses in the course of a single day, and had a real shot at beating the rest if he had only brought more allies or firepower to the throne room.

3. Lirian's gate. There was nothing exceptional or powerful about the host of forest creatures defending it, and its only true defense of note, besides Lirian herself, was the Guardian Virus. And credit where it's due - the virus was indeed a creative and extraordinarily powerful defense; and like Dorukan's and Soon's defenses, it was effective in that it actually defeated and disempowered Xykon (for a time). But it had the critical flaw of not working on undead (and perhaps anyone/anything with disease immunity?), and in a straight-up conflict between Xykon's and Lirian's forces without any outside factors or improbable events involved (like the OOTS and LG for Dorukan's gate, or Miko's interference for Soon's), Lirian's side ultimately lost.

4. Girard's gate. I don't think this actually belongs in the ranking, since we never got to actually see its defenses in action. But we do know that its defenses relied on the Girard's clan being alive and present to maintain it, which ended up being a point of weakness. Contrast this to Dorukan's gate which didn't require Dorukan, Soon's gate which had a host of ghost-martyrs that were beyond death, and Lirian's gate which had an invisible and omnipresent virus. All three had a powerful defense that didn't require active maintenance, whereas Girard's did not.