Well, as an ever-OWL, I mostly remember R4ge, the original WPK, and NASA as the ones we loved to fight against, because of the fun and constant arms race since we all had similar ages as corp, and then the reformed WPK and ESB we loathed to fight against, because we had really trouble beating WolfM squads with diving adaptive Guards, and had almost no good gunship skills for ages, and kinda refused to do it aswell. On the other hand, the really really few victories were awesome, especially when Tacklers got finally buffed to be usable.
It always felt weird to me, while WPK had troubles beating NASA, NASA always had trouble beating us (more since we had experience with them from late night pubs and probably ping on our side), and we had trouble beating WPK in tournaments, back in the really old days. I think, I learned most of these two corps and their individuals, so for me personally, they had the biggest impact ingame. The friendly matches where we all mixed and fought were possibly the best I can remember. Tough and exciting! Tons of fun.
When they left T3 mostly we stayed behind, with most members being casual, so I also remember many of the corps mentioned too, but since this was the time I started to transition up to T5 solo, I started to miss whats happening, and all these corps also changed and evolved. We did have our brief five minutes of fame later, when OWL exploded in size, and the fusion with Arch, which was exciting, but also kind of an experiment. There was this epic SecCon landgrabs, and stuff like that. And fun politics and getting forum bashed, and the first time we were seen as antagonists, instead of underdogs. It also felt not very owly.
I do see Ninja as worthy. Or Eagle / Sky. They were kinda underdogs, which sometimes surprised you out of the blue. It4ly mostly was iconic for being there, all the time, kinda like Ultra, and of course some individuals with high skills in pubs. Nova had some impressive reliability in battle, especially against ESB, but they lowered their guard more in a fight; while ESB just shot me out of the sky, it felt like, Nova was studying me before killing. I think various Nova pilots were the ones I followed to teach me how to survive in T5 meta, or realistic game mode as a soloer.
SRS and SYN have been around a long time aswell, but I only remember a few encounters, however of course I remember a lot learning from some of their individual aces aswell.
All that I mentioned here were usually in highest respect for me, and reading their insignias were a sign of a tough fight to come.
I think it is tough to say “top 15” objectively, we might all remember different times. Also everybody constantly has to exercise and evolve, and activities fluctuated over the years, both in corps, but also generally ingame. There were some really dark times (roughly 800 ppl online…), which sometimes broke corps, and in the end its a game, so people come, people leave, people come back. Still interested to see the result. And the follow up QQ.
Also of course, for me personally OWL comes first. Would be weird if not.