For me its Aquarium Note. I do tank maintenance service and it can track all my client's tanks. Saltwater, freshwater, and brackish tanks. I can set custom parameters. Line graph for each parameter. Log each tank's equipment and livestock. Set notifications for expendable media or when next to clean powerheads or pumps. Log daily maintaince. Share tank parameters thru email, different messagers. Export thru email in a HMTL or XML file. Take pictures of all tanks and critters. All for free.
Oh and it does a great job tracking my tanks.
The only thing that could make it better is if it could connect directly to Reefkeeper, Apex, Cerebra, Reef Angel, and all other controllers out there.