In my 30+ years in the collectibles market, that is the question i am asked most frequently. I have answered it many ways, naming cool, up and coming items from 1970s Star Wars action figures to hand painted Marklin trains from the 1900s. But as I get older and wiser, I have found a wonderfully evasive answer that happens to be the best advise I can give.
In an average year, I handle & graded over 50,000 toys & trains
What makes me somewhat unique in being able to answer such a question, is the thousands of hours I spent researching market values and collectible trends. In an average year, I handle & graded over 50,000 toys & trains, and have as part of my job, had to accurately attach a "current market value" those items. As a "perceived" expert in the auction business, bidders frequently asked, "What lots are sleepers?"(lots that may slip through the cracks and sell low), "What should I buy that will be a good investment", or my personal favorite, " What condition is this?" Just for fun, I will answer the above questions here:
If I did my job as an auctioneer, there will be no sleeper lots.
(Question 1)If I did my job as an auctioneer, there will be no sleeper lots. If you are asking me, I will direct you to lots that may have been late additions and may not have the exposure that others have had, thus allowing me to give those lots more exposure. Do you think, that you are the only person I'm telling this too? Knowing market value is the best way to notice potential sleeper lots during an auction.
If you base your buying on free advise, its' probably only worth what you paid for it.
(Question 2), The best answer to this, from an auctioneer's perspective(remember, he is working for the seller), is the lot he thinks will be the hardest to meet the consignors' expectations. From a buyer's perspective, the best investment is the item that will bring you the most joy or appreciation. As I have offered many times; make sure you really want the item, because you may have to live with it a long time to get your money back, if at all. If you base your buying on free advise, its' probably only worth what you paid for it.
"The only thing that cannot be changed is the way a collectible survived through history."
(Question 3), The 3 most critical factors in evaluating collectibles is Condition, Condition & Condition. The only thing that cannot be changed is the way a collectible survived through history. If a sealed item is removed from it's packaging, its no longer mint. If an item gets dented or scratched, it is no longer like new. If an item(automobiles not withstanding) get restored, it is no longer original and is worth less. In the world of toy trains, it is my experience that very few collectors have ever seen a truly Mint Boxed Vintage Example. In addition, it is rare that two collectors can agree on the accurate grade of any collectible. Ones ability to accurately grade is directly related to how many pieces they have examined. But giving up your decision to properly grade and relying on someone else, is a disaster waiting to happen and a direct path to disappointment. That said...
The best Investment in collectibles? The accumulation of knowledge.