Your choice

Those two aren't really comparable. The Sigma 10-20 is a rectilinear design (ie straight edges remain straight) whereas the Pentax 10-17 is a fisheye (ie buckets of deliberate barrel distortion).

A true comparison would be between the Sigma 10-20 and the Pentax 12-24; the Pentax lens is dearer and not as wide, however, I (among many others) was convinced it was worth the extra £££'s because of the superior results.

Sigma, Tamron, etc produce fine lenses "for the money" but given an unlimited budget I doubt many would fall into peoples' "dream" lists...

My turn I am slowly building up my dream kit, but this is how it looks

FA 31 Ltd
FA 50 1.4
FA 77 Ltd
FA* 85 1.4
Sigma EX 100-300 f4
DA* 16-50

To complete it:

FA* 200 f2.8
DA* 400 f4.5 or f5.6

Some Tamron and Sigma lenses fall into my list - ones that are different enough from anything Pentax makes (yet).

It will be interesting to see what the forthcoming Tamron 10-24mm will be like. Any good and it'll certainly be on my dream list. It would be great to have such a range all in the one lens. Meanwhile I like the Sigma 10-20. I would be dreaming about 10mm if I had a 12-24 instead! The image quality is not much inferior, by the way. The only major downside is that it only goes up to 20mm.

The Tamron 17-50mm F2.8 is also in my dream (and got already) list because of its low weight (for the type) as well as high IQ and speed. The nearest Pentax equivalent weighs a whacking great 600 grams.

Sigma 50-500mm must be fun - so that goes on the list too. Maybe also the 50-150.

Of course I would include loads of Pentax lenses as well, but I can't (and don't need to!) decide which ones now.

I don't need as much as you do ,

it depends which way I'd go (zooms or primes)but after playing
with Nathan's 31mm I'll probably go with primes,

so,

31 ltd
77 ltd

or

DA* 16-50
DA* 50-135

working with primes gives me A LOT more joy
from photography but if I'd take photographs as a professional
I'd go with zooms untill I would'nt be confident about my
manual-focusing skills .

Would love a DA 2.8 100mm Macro, cannot justify the expense at the moment

semi-realistically (the first time I did it I listed most lenses under 100mm and primes upto 600mm.. :lol
DA12-24
DA*16-50
DA*50-135

DA15
FA*24/2
DA*55
FA*85
FA100

Ive never really had a need for lenses >50mm before, hence the abundance of w/a