For me it was playing the great courses of the British Isles! Despite all the books I've read and collected and all the time I've spent doing research, there is NO substitute for being an eye witness to the great golf courses of the world! If I were to make a recommendation to those aspiring to enter the field of golf course architecture, study in the field as many golf courses, good and bad (but mostly good) that you can. It will pay great dividends for you down the road as you call back on those experiences and observations, etc. As you get busy, you will find it much harder to see those courses but you have to keep at it as there is no better way to learn.