I was warned that this astrophotography was expensive, for the amount of money I've spent I could have booked on ' Virgin Space' or whatever it's called!!!!
With little hesitation I would have to say it was Optec's FocusLock to use with my ONAG(s). The ability to image all night without re-focusing is pretty amazing. Lodestar owners + OAG can now take advantage of this with Optec's Lacerta.
A close 2nd is the SX-AO-LF. On my dual scope setup it has eliminated the problem of differential flexure between the two scopes allowing simultaneous imaging for up to 20 minute subs.
This is difficult to pin down to one thing. I've done a number of purchases recently to set up a new scope (refractor) that individually have aspects that qualify.
However, I think it's getting the Coronado SolarMax 60 DS that I'll pick - just because it opened up new opportunities for me. I can now image the Sun.
This year for me was getting Maxdome and TheSkyX all set up and working well with a Vixen Atlux / SkySensor2000-PC rig, and adding a ZWO 1600MM-C + seven slot filter wheel to take 36mm filters (L,R,G,B,SII,Ha, OIII) is I guess the bees knees!