As others have pointed out, this subject is a perennial. You can easily spend hours reading about it in many threads here.
I've stated my views and preferences many times in the past, but here, I'd like to observe that you have listed two specific goals: a smooth, dark tone, and enough projection for a good solo. Just keep in mind that with the standard French and quasi-French style classical mouthpieces, these two objectives are partly in opposition. That is, although you should be able to find a good compromise mouthpiece, the darker mouthpieces will tend to offer less projection, while the more projecting pieces will tend to have a brighter tone.
For example, the square-chambered Selmer S80 (typically a C* or C**) is rather powerful for a classical mouthpiece, but also somewhat bright compared to other options. To a lesser extent, this is also true of the Selmer S90. The round-chambered Selmer Larry Teal (LT) is a bit darker and a bit less projecting. The Selmer Concept offers a very good blend of tone quality and projection, but it's (i) available in only one facing, and (ii) quite expensive for a classical piece, so take those factors into consideration.
The Vandoren Optimum AL3 produces a lovely, dark tone, but its very narrow tip opening makes it rather constricted. Frankly, it's probably not the piece you want for large-ensemble solos unless you are a seasoned classical player who can really blow through it. I prefer the AL5, which sounds almost the same but is more powerful, and thus a better-balanced mouthpiece, IMO. The AL4 uses a different facing curve from the AL3 and AL5, and as a result is significantly brighter, more like an S80, although it still has a round chamber.
FYI, my standard classical mouthpieces are the SL3, AL5, and TL5. I have also gotten good results on alto from the Concept and the Larry Teal. Nowadays I don't ever play the S80 for classical or concert band work; rather, I treat it as more of an all-around mouthpiece that can work for jazz/pop styles as well as light classical.