Well as you say, there have been so many signings in "our history" that I am still dubious he would be even in the top 5. If we are talking very recent history then sure. It would be interesting to see who would come above Lambert since, let's say, 1976 (can't imagine why I chose that date). Some names off the top of my head that could possibly be argued as better signings for various reasons:
Keegan
Shilton
Pahars
Case
McDougall
Boyer
Any others?