I am often asked what mouthpiece I use, and for many years the answer was the same; 'a cut down medium bore euphonium mouthpiece' (actually a Denis Wick 4am). In fact when I left England bound for New Zealand I bought extra ones to bring with me, and because they were 'turned down' along the shank to fit the baritone leadpipe, this made the shank thin and easily damaged. Needless to say I went through quite a few of these over the years.
Then Steven Mead brought out the 'Ultra' range and for the first time in over 20 years I changed mouthpieces. For me the Steven Mead Ultra baritone mouthpiece gave me the extra projection and 'weight' I felt was sometimes lacking with my existing mouthpiece, (especially when competing in solo competitions against euphoniums!) whilst letting me maintain my sound, range and articulation.
The 'Ultra' compliments the Besson Prestige baritone superbly - in fact it was great with the Besson BE955 3 valve instrument too.
So when I am asked what mouthpiece I play or recommend for baritone players these days, for me it is the Steven Mead Ultra or Ultra X, dependent on your needs....and no I don't get paid to promote them :-)
Of course mouthpieces are a very personal thing and you need to find what works for you. My advice is try as many as you can (if you have the chance) - you'll work out fairly quickly which ones don't work for you and which ones you maybe want to try a little longer until you settle on the right one.
Good luck.
Steven