So I guess I'll chime in here too since I'm also a veteran UW fan. Been listening to them since 1995 when Cowgirl was featured in the film Hackers. I wasn't terribly into techno at the time, but I had always been a huge Depeche Mode & Kraftwerk fan, so falling in love with the sound of Underworld was obvious.
My love for Underworld really didn't come until later when I really got into Pearl's Girl being featured in The Saint. At that point I was finally introduced to Born Slippy by a friend and then I obtained STITI. Instantly became a favourite of mine and still is by far for me their greatest LP. I really can't say that I have a least favourite because each LP has 1 or 2, and in some cases 3 of my favourite UW songs.
For me, there is something about Barbara Barbara that really moves me and gives me a really positive vibe. It's like finally reaching the light at the end of the tunnel by the time Nylon Strung starts, which for me is one of my favourite UW tracks now. I even have my wife and kids humming along to it, which makes the emotions run high.
My 2 daughters (4 & 10) had me repeating If Rah today. I love it!