The answer here depends on what you mean by "street magic." If you're talking about busking, then the "best gimmicks" aren't on that list.
If you're talking about just walking around doing magic on the street, then you're really talking about close-up magic (geez... I wish people would just say "close-up magic" instead of "street magic" when that's what they really mean
). Close-up magic done on the street is not "street magic"; it's close-up magic done on the street.
The Raven is a one-trick pony. Meh.
IT is very problematic when used close-up. Too many risk factors involved to make it "best," though when it works, it works well.
TT... definitely the most versatile items on the list. If you keep in mind that it wasn't designed to be used as a close-up gimmick and ensure that you put in the proper work to account for that, this could be #1. However, you need to a lot of planning, routining, and pocket management if you are going to do more than one trick involving a TT in the same routine. Can be as problematic as a Raven in that regard, unless you carry more than one.
Scotch & Soda has reset issues if you use the standard set...which is why I use normal, ungimmicked coins to do this trick. To clarify, Scotch & Soda is a trick, not a gimmick; the trick can be done with a standard Scotch & Soda set, which includes a gimmick, but it can also be done with normal coins.
Sponge Balls. Very versatile, though if you're going to do stuff on the street, you'll want to make sure you have in-hands material in case you don't have access to a clean working surface...or any working surface at all. Have to say, though, that a sponge ball is not a "gimmick," so it doesn't belong on this list...unless you change the question to what is the best "prop."