I have recently finished a contract with a client that has taken at lot of my time over the last 18 months, they are a private medical group based in Singapore that performs stem cell replacement therapies, their site is at http://www.globalstemcells.com - which was a wonderfully dynamic Drupal 7 site until 3 months ago when they decided to trash it completely and build a Wordpress site with a purchased theme, always sad to see :(
I answered a request for help for a "slow site" in a post in a forum; they liked my proposals and I then spent the next 18 months working on various aspects of their site. The initial slow loading problem was the result of a rather liberal and wide open user registration page, they had 250,000 user accounts registered by bots, they were all continuously posting comments that were filling up user account tables, moderation queues and logs; it was a mess.
The sites hierarchy was also adding to the database size with a multitude of single use content-types created with a whole host of unused fields attached; I think they had 38 content types in total - after the eventual rebuild we were serving the site with 5 content-types, this helped to reduce the database size ten-fold.
This site presented a number of challenges mainly in the area of best-practice and common configuration; they seem to have got a less experienced developer to build it initially, so many things were just wrong!
After the initial site stabilisation I then produced a new theme for them based on designs created by their own graphic designer. Initially they were passed as single view desktop designs; they were very open to the ideas of mobile-first and allowed me to interpret the desktop design the best way that I felt they should react in a responsive viewport; I prefer to design responsive sites in-the-browser as often graphic designers just don't seem to understand mobile sometimes.
Upon completion of the theme I was then challenged with fitting out the site for adding translations for 15 different languages for every page, menu and item within the site. This was quite a major undertaking bringing in a wide array of translation modules, display helpers, Views add-ons with numerous challenges along the way. I then spent a few months being on-hand for them and their team of translators working on the site and then pushing content live as it was translated.
Eventually though, their organisations internal pressure to align their web presence and bring everything in-house, signalled the death of their Drupal site; such a shame. They have 8-9 other domains they use that all feed into the same business, the main site was one of their oldest but they now had an in-house WP developer that wouldn't touch a Drupal site.
I enjoyed my time on their project as they were a great bunch of guys with some good ideas about site delivery and content construction; quite a good marketing team they have there.