Bass clef generally. The Tuba is considered to be a non-transposing instrument, so a BBb, CC, EEb or F tuba will read from the same music and it is up to the player to know the correct fingerings....
The viola is written in alto clef because that is its musical note range. Note that you are required to learn "the violin clef" too. Also because we needed another clef... and violas are special and...