I think that Sheik is the best character in the game without advanced techniques. I think Marth is the second best character in the game without advanced techniques. I also think that Sheik benefits the least from advanced techs out of every character in the game. If not the least then pretty close to it. I think Marth also benefits slightly from advanced techs, but not nearly as much as other characters.
Fox and Falco are only as good as they are because of advanced techs. The same can be said for Peach, CF, IC, Bowser, Link, Ness, Ganon, etc.
Therefore, I think that Fox and Falco ranking above Sheik and Marth in the tier list is quite accurate. Think of it this way, Sheik is an 8 without adv. techs, Marth is an 7.5, Fox is a 6, and Falco is a 6. When adv. techs are implemented into gameplay Fox becomes a 9.5, Falco becomes a 9.5, Sheik becomes a 9, and Marth becomes a 9.
Sheik is the best character without adv. techs. (period) With adv. techs, she is not, Fox and Falco are better (overall). However, I think that Fox vs. Sheik and Falco vs. Sheik are pretty much 50/50 matches. Without adv. techs Sheik murders them, but with adv. techs it's an even match.
Discuss.