OK.. I know it's childish, but hey :)
So, which of the five is your favourite?

Thunderbird 2, no question. I had the toy when I was little and it had the bonus of a little Thunderbird 4 so that was great. I played with it till destruction and have now started a weekly ritual of kicking myself now I know what a model in mint condition is worth. Don't get me started on how someone snapped the aerial off my Captain Scarlet SPV.

Plus I always thought Virgil was the man.

Thunderbird 2 - it was the most important, with the obvious exception of TB5. An example of its importance is that it appears in 31 of the 32 episodes and the episode it does not appear in, "The Impostors" is because it is a space rescue.
It looked the most ridiculous...but it had an anarchic beauty.

I remember someone asking if it would be possible for a TB2 shaped plane to fly, even with the pod missing.

I agree, Thunderbird 2 was and still is my favorite, with its power, curved lines, and multi-function cargo holds.
My second favorite is a tie between Thunderbirds 3 and 4, then Thunderbird 5, and last, Thunderbird 1, but only because it's the closest to a common rocket.

I would have to say designwise it would be
Thunderbird 3 because it looks so sleek.Otherwise it be Thunderbird 4 since I like underwater exploration even better than deep space.

Thunderbird 2, all the way. It does most of the work, after all. Looks like a very fast blimp, but even so..."Terror in New York" proved just how important TB2 is.

Plus...it's piloted by Virgil, who is definitely the man. (This thing I have for heavy eyebrows....)

Thunderbird2... same reason as DK ;-) although I'm not a big eyebrow fan ;-)-- love the hair though!

I guess TB1 and TB3 are both rather unextraordinary rocketship sort of thingys. At least to me they are. TB4 is cool, but the design isn't all that unique for an underwater vehicle.

I love the fact that TB2 has pods! It's big and green and can be maneuvered with reasonably high precision. Whether that's due to Virgil's skills as a pilot or if he's got some computer help-- I'm always impressed with it's flying abilities. Also, because TB2 is such a "multi-purpose" vehicle, I don't think IR could really do any serious rescues without it. Gosh, I remember my brother and I fighting over who could pilot TB2 when we were small kids...*sigh* the memories of my childhood...

Thunderbird 2 and Space:1999's Eagles seem to be very similiar in terms of functionality.

First post on the Thunderbirds board here as HMV were selling 7 of the DVD's at £1.99 each.

Toss-up between 1 and 3- but probably 3.

Unoriginally, 2. Also Gerry Anderson's own favourite Thunderbird, apparently.