The Magnificent Seven Justice League of America
in order of their original joining-up
I will probably get hate mail for this one, but let's face it: Aquaman has super-strength at a reasonable level (weaker than Wonder Woman but stronger than Batman), so if we want actual team-work to occur Aquaman is our guy. In his current series it's been shown that he is nearly bullet-proof and can also leap over buildings in single bounds. Personally, I would like to see him portrayed as similar to Marvel's Captain America or Namor the Sub-Mariner: no slouches in the power department. Besides, the Earth is mostly water. When aliens attack the oceans are going to have to be involved. Aquaman is in.
Not only is The Flash fast enough to atten all meetings and fight all the bad guys and then still return to his secret identity with time to spare, Barry Allen (and then Wally West) were both fun personalities to have around. As long as he is not portrayed as extra-super-ultra fast so that he can do absolutely anything, he's a great character. And I don't know if it's just me, but I always thought The Flash was one of those characters who works better in a group than on his own.
I don't really care if it's Hal, John, Kyle, or even Guy; the Justice League needs a member of the Green Lantern Corps on its roster. They need the power and the window to all sorts of space opera that a cosmic policeman would bring to the series. As for which one to use; each of these characters brings something different to the group dynamic. I was a die-hard Hal Jordan fan for my first years, but in the past 20 years or so I've very thoroughly joined the John Stewart camp. Guy Gardner is better as an antagonist, and not really appropriate to The World's Greatest Super-Heroes.
I would include Wonder Woman in the mix for some of the same reasons I included Aquaman: she is strong but not in Superman's range, so would logically need help from time to time. Similarly to the Flash she's a very dynamic and exciting character but somehow more interesting when playing with others rather than out on her own. I want to like Wonder Woman, but I just can't read her solo series. I love her on Super Friends and in the Justice League, however.
Each group needs a bit of "personality," and as he has been portrayed for nearly 50 years Oliver Queen has personality to spare. Plus without spotlight-hog Batman around, he can also function as the "stand back and plan" tactician type guy. Because, really, an archer works best when he has his eyes on the overall situation, not leading the charge.
Black Canary is one of the first "legacy" characters: her parents were crime-fighters and she grew up being watched over by their friends the Justice Society. She is an awesome hand-to-hand combatant, and her sonic powers (when handled correctly) are formidable. The fact that she's got long blonde hair and fishnet stockings in her costume is just a bonus.
The last spot is always the hardest to fill because each of the remaining candidates bring something different to the group, changing the dynamic of the series. If I went with The Atom the series would have more of a scientific slant. If I went with The Elongated Man stories would tend more towards the comedic side. If I went with Red Tornado, his ongoing effort to come to grips with his own type of humanity would definitely be part of the series. And if I chose Zatanna then the series would have a permanent link to sorcery. Obviously, in the end I chose Hawkwoman. Shayera Thol can lead the stories into both science fiction space opera as well as straight super-heroic action, and also into mysteries and horror. And instead of her husband, who could be a supporting character if the writer was so inclined, having Hawkwoman as a member gives us three female members in the group. Plus I would like to know if her political views are as conservative as her husband's; if so, she and Green Arrow can get into the same type of arguments that he and her husband used to get into.