Having a very fast UX for data entry seems like the most important requirement for me to ensure I'll actually track stuff and not feel distracted. Opening / editing a spreadsheet or some app just takes too long and if it is an app it forces me to think in its paradigm.

So I set up a global hotkey on my mac via icanhazshortcut that opens/generates a text file in Sublime Text with the current date in the filename and set up a hotkey within the editor to insert a timestamp. I've established a syntax that's basically [keyword] [number] that I can use for tracking food calories, fitness, etc. I can record I ate a couple of yogurts via just two keyboard shortcuts and typing 'yogurt 2'.

And now I'm building an app that reads and processes those files and puts stats in a database or exports to some app for analysis (just the former for now).