Some of the DofC notes such as 1870 $1.00 or 2.00's
train notes
higher denomination 1935's in EF or better
rare replacements
-These have always attracted a lot of attention, and are a pretty safe bet. However, you need deep pockets to get them in the first place, and this is assuming that collectors will turn to the harder/scarcer notes to acquire. That is the problem, for instance, with some of them (for example the train note, $25 1935 or * DF).
If you think about other notes that have performed (increased in value) over recent years, look to DF (oh & if only you could find an * in any condition!), DF change-overs in particular, Osborne-Tower 37's and change-overs (eg: $10 G-T Z/D), and (as you listed) the rare replacements, though I think there's something to be said to collect even the less rare * asterisks as new info makes such formerly inexpensive buys (ie: *B/M with no fpn/bpn) double in value.
I am probably one of the few who believe there are 1969 multi-colured (esp "good-overs" & test notes) that are under-valued and even a few Birds. I also believe that many errors are under-rated/valued at the current time and this will change (however, most people who find them tend to over-inflate or over-estimate their value rather than let the market decide).
I tend to watch eBay, here, and other sites to try to get an idea of what is selling a lot and what is something we don't see too often. I also think it is wise to jump into some of the rarer recent issues (change-overs/short perfixes) - & when you choose to "cash in" is always a hard call. I would never criticize someone for getting a good return on their notes (as an investment) esp when they may be changing the focus of their collection (as I've done many times!)
