As I have written in another post, I started to play bass clarinet. I got a Yamaha 221 II bass clarinet with a Selmer Concept MP. While I love the fullness of the sound of the Selmer MP, it's not that easy to play in the higher registers for me; I also didn't like the enclosed Yamaha 4C MP, it's too narrow for my taste.
So I tried many intermediate tip opening MPs and ended up buying Yamaha 5C, which I found better in sound and playability than any other within the tried range. While I might outgrow it some day, I think it's really a serious mouthpiece (much better than 6C for me, which felt narrower and had an incredibly boring sound).
Probably there are quality fluctuations to be taken into account, but for the price I think you get a really serious mouthpiece with the 5C.