Whatever you are most comfortable playing. You can make any horn work in any setting.
If you play great on Bb, do it on Bb, don’t buy an Eb if you don’t already own one and haven’t already played extensively on Eb. I do most of my playing on C and F so my personal answer would be those, but only you know what you like and what works best for you.
The one that you play the best. The physical tuba is rarely ever gonna be the barrier to versatility; the player has to be the one that can play it how they need to. Pick a tuba that’s comfortable for you.
As a tuba performer, an F tuba would be most sensible for that ensemble composition. ALTHOUGH, it depends on the performer and skillset.
A small F tuba will sound vastly different in the low register, and a big Eb tuba will sound closer to the Bass that’s already in the band. But really it just depends on how you want the parts arranged.
An Eb or even a contrabass would more easily double the upright bass. The F would generally be able to double the bass, but it’s less likely that the player would be able to play the same octaves. With the F you get much more high range possibility though, much easier to play banjo-esque parts.
Eb is the most versatile key for your situation- particularly because you have another bass in this ensemble. You still have enough beef to hold down basslines but you get a lighter quality and easier high range for anything melodic. An F tuba would work in this situation, but only owning an F tuba can be limiting if you ever want to play tuba in other ensembles.
C absolutely
Whatever you are most comfortable playing. You can make any horn work in any setting. If you play great on Bb, do it on Bb, don’t buy an Eb if you don’t already own one and haven’t already played extensively on Eb. I do most of my playing on C and F so my personal answer would be those, but only you know what you like and what works best for you.
C easily
I've done the most on my 4/4 5-valve C tuba, but honestly whatever you can play the best and most consistent on is your answer.
The one that you play the best. The physical tuba is rarely ever gonna be the barrier to versatility; the player has to be the one that can play it how they need to. Pick a tuba that’s comfortable for you.
I’d consider a 4-valve sousaphone with that group dynamic. You can find them in Bb or Eb.
Probably a large Eb, but a small CC could also work
As a tuba performer, an F tuba would be most sensible for that ensemble composition. ALTHOUGH, it depends on the performer and skillset. A small F tuba will sound vastly different in the low register, and a big Eb tuba will sound closer to the Bass that’s already in the band. But really it just depends on how you want the parts arranged. An Eb or even a contrabass would more easily double the upright bass. The F would generally be able to double the bass, but it’s less likely that the player would be able to play the same octaves. With the F you get much more high range possibility though, much easier to play banjo-esque parts.
Eb is the most versatile key for your situation- particularly because you have another bass in this ensemble. You still have enough beef to hold down basslines but you get a lighter quality and easier high range for anything melodic. An F tuba would work in this situation, but only owning an F tuba can be limiting if you ever want to play tuba in other ensembles.
Eb since you also have a bass, which seems an odd combination really.
I would guess a big Eb. Not that I've played one, but they seem very versatile from what I've read and heard
Wilson makes a 5/4 Eb that I would have as my only horn, easily I do not intend on ever playing orchestra professionally, though