Serra's Sanctum is an old-school powerhouse, in the cycle with Gaea's Cradle ($150 for some time now) and Tolarian Academy ($25 for ages, due to restrictions and bannings). Serra's Sanctum is probably not as good as either of those cards, but it is very, very good.
I've always wanted one, it's on the reserved list and it's seeing what might be a doubling up right now.
This perfectly fits the bill for a long-term MTG investment. A sizable chunk of value all in one card, Reserved, ultra-powerful, simply no way to print a better version (it's too good as is), and gets better with time.
Get in on these around $25-$30. I think a value of $50 by Jan 1, 2016 is likely, and don't be surprised if this has 5-10 year legs on it, either. I can see these as $100 cards a few years from now.
Also, keep a close eye on Tolarian Academy; that card is a dormant financial volcano.