I have Navigon, Tomtom and Co-Pilot. I previously used sygic.
To summarise:
Navigon is best for long distance traffic on A-Roads upwards and voce directions. It includes TMC traffic and some flow information with a range of about 100-150 miles. Routing can be strange and ETAs are generally overly optimistic (i.e you'd need to drive well over the speed limit to make the times they say). It also has a function to read out traffic info (giving location, reason and expected delay).
Sygic is the prettiest and voice directions are decent but routing and ETA is poor. They have been promising an update to use Tomtom's IQRoutes for about the last year but I wouldn't hold my breath. I haven't used Sygic since they changed the traffic from Inrix to Tomtom though. Traffic uses the same info as Tomtom but without the IQRoutes info, it's less comprehensive.
Co-Pilot is the most customisable. You can set your own road speeds etc to optimise routing to your driving style. It uses Android's native TTS though and this is nowhere near as good as Sygic, Tomtom or Navigon. Traffic is similar to navigon, with a shorter horizon.
Tomtom is very solid for the money. The voice direction is a little functional compared to Navigon but it does the job. Routing and ETA are about as good as it gets, the best I have used on a satnav. ETAs are typically within minutes for several hours driving (they use measured rather than assumed road speeds). Traffic is more detailed compared to Navigon or Co-Pilot, in that it includes B Roads and some other 2ndary roads , plus traffic jam positions are often accurate to tens of metres. Live Traffic is generally excellent but has two main flaws. One is that it doesn't include all TMC info that you'd get from RDS-TMC on a standalone TomTom or Garmin unit (Navigon and Co-Pilot do). Say you've got a trip over the pennines with potential snow on A roads. Frequently Navigon or Copilot will know the road is closed but Tomtom will send you happily towards it unless there's a big jam building up. This is inconsistent, depending on location. Tomtom traffic is fine for Highways Agency Trunk roads but closure info from local authorities is very hit and miss. The other flaw is the range. For traffic flow and general incidents the range is 40 minutes. For road closures it's 160km. 40 minutes can sometimes mean it's too late to select the best long-distance alternative by the time traffic is detected.
Overall I would recommend Tomtom or Navigon. Tomtom for traffic avoidance in commuting and medium distance journeys, routing and accurate ETAs. Navigon for superb voice direction, that's so good you usually don't need to look at it, and better info on road closures on A-Roads and Motorways.
If data's not a worry, I'd say the best freebie is M8. That's free, includes the same live traffic service (free) as Navigon but it all runs online.
If you don't have a data connection, there's not much wrong with Navfree.