The three founders are essential.
1- Cosmic Boy
2- Saturn Girl
3 - Lightning Lad/ Live Wire (I like the modernized names. Most of them)
Next, we have two characters that, IMHO, are essentially linked to the team. The brains and the shape-shifter/spy).
4 - Brainiac 5
5 - Chameleon
Then, muscle. You have to have a brick. In the Legion's case, a Superboy type. This one is also my favorite Legionnaire, especially when they bring his street smarts into play.
6 - Ultra Boy
An extra powerhouse would be intresting. This one is realtively new, with unexplored potential, and, IMHO, really intresting.
7 - Kid Quantum (the female one with inborn powers, of course)
After that, versatility and more power.
8 - Apparition
9 - Shrinking Violet
10 - Colossal Boy
11 - Invisible Kid (Lyle Norg. I like playing him of Brainy)
12 - Inferno (I prefer the male one, but I'd be okay with the more disruptive female one)
The Next 12 members would bring the team up to it's true power potential. I prefer to have members that reflect more an allien diversity, rather than just changing skin colors.
13 - Gates (love the allien enemy of the system)
14 - Wildfire (the original)
15 - Shikkari (I prefer her to Dawnstar, mostly due to her more allien nature)
16 - Mon-El
17 - Kinetix
18- Sensor (The half human, half serpentine version of the later part of DnA's run, though I'd prefer a personality similar to the one she originally had, rather than the later angry one. Also, give her mystical knowledge and abilities.)
19 - Timber Wolf (a personal favorite)
20 - Light Lass
21 - Umbra ( I loved the more "savage" Umbra of the POst-Zero Legion, especially her ability to use her dark fiels as a physical force)
22 -Element Lad (the original)
23 - Triad (each with her own personality, such as in the Post-Zero Hour Legion. As powers go, may not be much, but makes for an awsome character).
24 - Karate Kid
Peace