Fox is much more consistent than Falco. Fox has more tools and options at his disposal and is definitely better in teams. I would even go as far to say he's better in singles simply because he doesn't choke and get capitalized on by every single mistake like Falco.
Falco has spurts of momentum, but chokes hard when he's not doing well. Fox is more well-rounded IMO, and doesn't do as badly on somebody's bad day. It sometimes really comes down to preference, though, whether people prefer to play a safe and consistent character, or a risk vs reward character.
I think Marth and Sheik have a ton of potential, but as you said, nobody's been using it, and that could be a sign. Right now, I'm willing to say Fox > Falco > Sheik and Marth as many others are.
see thats my whole point though.
fox is in theory a better char but isnt the tier list supposed to account for the top level of play?
top level falcos consistently outplace top level foxs. furthermore, falco gets messed up when he's not doing well, but falco has longer gaps in between doing well and doign poorly because of his ability to control spacefrom anywhere on the map.
and i think its a misconception that fox doesnt get capitilized on as hard as falco. Lets face it, if you're playign fox or falco, it doesnt matter because any mistake you make is gonna lead to a long *** combo and probably your stock regardless. the difference is falco has a gun which prevents his opponents from always doing what the want, so falco has longer stretches where he's in control and because of that, he gives his opponents less chances to capitilize on average than fox.
I guess what im saying is, there are 3 general situations in smash:
1. you and your opponent are in a neutral position and neither is hindering/affecting the other
2. You are comboing our opponent
3. Your opponent is comboing you.
you win a match generally by using spacong/mindgames involved in number 1, to increase your likelihood of number 2 more than having the reverse happen.
With fox, all 3 scenarios occur at roughly equal proportions
Falco is the only char in the game where 1 is almost never happening. His laser affects the match such that either 2 or 3 is the only situation occuring and because he is the one shooting the laser, more often than not, option 2 is what tends to happen most.
in a nutshell what im saying is falco's ability to control space from anywhere basically means that he has a hightened chance of gaining a advantageous position over pretty much anyone and thats why he's the best char in the game imo