Accurate: Hg in glass, 76mm column, NIST traceable. Expensive, hard to read, easy to break, hazardous when broken.
Alternatives: VWR.com, fisherscientific.com and all of the fish supply vendors carry cheap digital thermometers that vary in terms of accuracy. The lab supply sites will tell you lots about the accuracy of many varieties, from keyfob types up to NIST traceable ones.
Any reasonably priced one should be OK for aquarium owners. I have a keyfob type and a pen type - both are cheap thermocouples that read to 1/10ths in °C or °F.