Like all these lists - not really the most imaginative approach from the journalist, Jake Kerridge - there's no definitive answer and that creates often some great postings in the comments section where fans of different authors argue over which deserve their place and which not.
Len's work Billion Dollar Brain is Kerridge's surprise choice (surprise for me in that, most journalists will often pick Funeral in Berlin or The Ipcress File from this series). There are some other well-deserved choices but, as some commentators remark, where is Tinker, Tailor, Soldier, Spy?
It's all very subjective, and I guess that's part of the fun. So ..... what would you change?