Another vote for bass clarinet, but it's tied, for now, with the clavichord.

I would have to say the viola. It does what it says on the tin.

If we've delved-off into the world of "favourite obscure instrument", then I'll express my fondness for the Ophicleide.  Although it's now consigned to the footnotes of musical history, it was - prior to the development of the tuba and euphonium - a serious brass instrument, and played the bass lines in most of the C19th symphonic repertoire before Wagner's time. (Properly it should be used in Berlioz's and Tchaikovsky's works instead of the tuba).

It's an entirely viable solo instrument in the hands of someone who actually cares about it, such as Ollie's countryman Nick Byrne:

(sound samples)
http://www.ophicleide.com/articles/Audio.htm

Reiner, are white gloves de rigeur for operating the ophecleide? 

Tony, next time my contrabass clarinet comes to London you're welcome to have a tootle on it. Ian will know when that is. (Not my trip next week, I'm afraid, but almost certainly some time in the next year or so.)

Speaking of contrabass clarinet in The Avengers, I suspect I've used that very instrument for a recording some years ago in London - it had the name of the late Stephen Trier on the case and John Coppen was of the opinion that both he and that instrument had been responsible for the BBC uses of the contrabass in that period (Doctor Who as well, for example).

For those without Avengers DVDs here are some other places to hear the contrabass clarinet.

Richard Barrett: interference
Donatoni: Ombra
Grisey: Anubis, Nout

Unfortunately the Donatoni is out of print and the Grisey is performed on the wrong model of contrabass clarinet which can't do some important things the composer wanted because of a different mechanism. And I would have, er, a conflict of interest in recommending the Barrett because, er, it's played by me. Under a pseudonym of course.

So maybe The Avengers is still the best bet... 

Thanks, oliver, I'll try to get to London and take you up on that!

As for Avengers episodes, pretty much most of them as I remember. I remember being struck by how low the clarinet was, knowing that my bass couldn't manage that. I have got some Avengers DVDs (is that an embarrassing thing to admit to? - they were left to me by my brother) so I'll check them out. I've a feeling the black and white ones (with Dianna Rigg) are the more likely ones than the colour ones. Try BBC4 on Thursday at 7.10pm.

I did spend about half an hour last night flicking through Avengers episodes on a DVD to try and find something suitable (has my life really come that? Is that what I was put on this earth for?) but with no success. However, there are some very juicy moments to be found - trust me.

I can't watch it this evening because of other commitments and I can't record digital TV.

I just discover that I have no clue what is cor anglais. I am listening to the afternoon concert from last year festival in Lucern. They are playing a pice by Hollinger for cor anglais solo and with harp. When he started to play I understood that I don't know what he is playing. I know oboe and its sound. I am very ashamed. On the picture they look like oboes to me.