Odometer(total miles)
Distance of current ride
Ave speed
Clock

After that it's pretty much overkill.

I will use my smart phone to track stupid things like amount of climbing. But I'm doing a ride here shortly with 8,000 feet of climbing and wanted to know what I was doing on my training rides.
That's another thing. If you have a smart phone you may not need a cyclometer but I like seeing the cyclometer and WILL NOT mount my smart phone on the bike.

Cadence/schmadence....

Schwinn cyclometer from Walmart will run a huge sum of $10.