The Holy Grail has good reputation. A good, basic feature set with the classic spring reverb as well as the infamous "flerb" if you want to get a little weirder.
I had the nano version on my board for a few years before replacing it with the TC Electronic Trinity Reverb. It was pretty good for the size, price and general sound quality. I found it to be a little noisey and buggy to use. It would cut out unexpectedly and without warning. So I may have just had a "dud" pedal.
I would still definitely recommend this as a good "starter" reverb pedal though.