What Instrument Should I Get?

Bb Clarinet
If you want to start out on a regular Bb clarinet to get a feel for it, then the best (and cheapest) one that I have found is:

Buffet B12

Approx. Prices:
500 US Dollars
250 GB Pounds
450 Euros
(although cheaper online or second hand)

Made from ABS resin.
Standard student instrument.
Popular in schools.

Bass Clarinet (beginners)
The best bass clarinet to start out on is:

Yamaha YCL-221 II Bass Clarinet

Approx. Prices:

1800 US Dollars
1700 GB Pounds
2100 Euros

Also made from ABS resin rather than wood.
Low Eb.
Similar to the Jupiter JCL-675N Bass Clarinet which is a little bit cheaper.

Bass Clarinet (professional)
The greatest and most expensive coveted bass clarinet available is:

Selmer Privilege 25P

Approx. Prices:
14,000 US Dollars
7,000 GB Pounds
9,000 Euros

Made from grenadilla wood.
Low C.
A beautiful instrument.

A Word of Caution
Don't buy a dirt cheap no-name instrument: the keys will bend and it will sound out of tune!
Buy a Buffet, Selmer or Yamaha.
Buy from a reputable supplier.

Starting Out - Basic Tips

Are you starting from scratch?
Buy a regular Bb clarinet. Try it out. Use it to learn the fingering positions and the embouchure. Regular clarinets are a lot cheaper, easier to carry around and you can easily sell it later when you upgrade to bass clarinet

Do you want to double?
If you already play the saxophone or the Bb clarinet then the transition to bass clarinet is much easier.
You can go to a store, try out a few instruments and see how they sound. You can also buy an instrument straight away and apply a lot of what you already know.

Why do you want to learn bass clarinet?
If you want to learn how to play the bass clarinet so that you can get girls then forget it - buy a saxophone!
If you want to play bass clarinet to look cool then forget it - buy a trumpet!
If you want to express yourself creatively in a musical way and you love the sound of the lower register then learn to play the bass clarinet.