Easily 2009. LOVED the theme and the specially painted boxes, as well as the ice-cream stall for the 5-box twist. It had at least 3 tense games, including Callum's classic game and £35k Steph's game (the latter of which, people forget about), Irene's unavoidable trainwreck game was really the only waste of time. Overall it was great week and had quite a few talking points on here, Then there was the Newbie Yvonne that week who we never got to see dressed normally, as she was kicked off the show over the summer break and didn't return for the start of Season 5!
I also enjoyed the 2008 summer specials, but it was kind of ruined by Em's £120k Proveout offer and William's deal and banker bonus (probably also the fact that it was during one of DoND's worst peroids, it was like an oasis in the middle of the desert!)
2012's summer special - Well, I liked the theme. But the catapult ruined it, and it didn't really have any memorable games apart from Mark becoming DoND's unluckiest player
2011 was ruined by the catapult, but had a couple of great games in the form of Alphonso and Barnaby
2010 was meh, there wasnt really any memorable games apart from the late Laura who sadly passed away later (Sadly, that's probably the only reason her game is well-remembered

) and perhaps Dai whose £10k win felt like a lot more
I didn't physically see last year's but I liked the players dressing up as pop stars theme. But it was another pretty disappointing week in terms of memorable games