I think the 4th one from the left looks best and is the most varied. The others have pretty similar designs to the others in their trio, which is fine since they're different groups of legendaries.
Gen 2 looks pretty cool and gen 3 is very unique, but doesn't look that cool.
Gen 1 is the original and has two of the nicest looking legendaries (Moltres and Articuno). Some of the later gens wouldn't have contrasted that well with Mewtwo, so wouldn't suit Gen 1 so much. Apart from possibly gen 2, gen 1 is the only one where you can easily group them into something normal people would understand by saying "legendary birds", whereas the rest are all "legendary pokemon" or "legendary [word only a fucking weirdo knows] pokemon", which might make them more well-suited to being the first set. If Gen 1 came out in a much later gen instead, I don't know how they would be received. Some might say they're just unoriginal, but then again Zapdos and Articuno are fairly original in fiction (inb4 someone mentions some similar beings in some fictional piece 99.5% of people don't know).
