I could only pick one, so I had to pick speed.

Time can be taken care of by noting when I leave home and when I get back.
Distance can be found through a route-planning software I have.
Total distance could be summed up by keeping a record of all ride distances as calculated by the route planner.
Cadence can be calculated using speed and knowing your gearing.
Average speed can be calculated using total time and distance for a ride.
Maximum speed can be remembered "manually", provided you have the presence to look down at the computer right at the fastest moment.

So, that only leaves a need for instantaneous speed, which is the only computer function I'd really need, if it came down to it.

And it's the only function I use when I actually ride. Especially in city traffic, as I take care to avoid speeding through certain 30-km/h sections.