My vote would go to the central Devon route.
- It would link Tavistock, Okehampton and central Devon into the rail network
- It would provide greater resilience to the local network in case of works betweem Plymouth and Exeter
There's a debate as to how useful the second point is, however that doesn't mean that the route isn't without other benefits which are probably of greater importance.
Firstly, it's cheaper than building a DAL, which although does provide journey time improvements doesn't really provide much extra capacity to/from London due to constraints at the London end.
Conversely the line through Okehampton would create a direct service to Plymouth from the WofE Line, creating a shorter and quicker journey from a large area across Southern England, including Southampton, Portsmouth, Brighton, Weymouth.
By reducing the need for those passengers to travel via Reading/Westbury it would allow extra passengers to use the GWR services. Given that many of those passengers would be joining the WofE services as they are starting to see passengers get off at they travel out of London, that would mean that the cost of extra capacity would be fairly limited.
Whilst there's limited extra scope for extra capacity on the existing trains, it should be noted that following Crossrail 2 there's then extra paths out of Waterloo which then could be run along the WofE Line. Whilst an increase in frequency would require additional loops/redoubling of the line, that would likely be required without the extra Okehampton/Plymouth passengers anyway.
Whilst journey times from Waterloo would never compete on time, from Clapham Junction it would. Especially if there was a move to local services some of the note minor stations (could still have a direct London service every 2 hours, with each station being served by a different fast service, for instance on 2tph the 4 stations between Exeter Central and Honiton could have a direct service every 2 hours but still remove 3 stops from all the services as the between stations would be picked up by the local services run by GWR).
Not only would it improve longer distance travel options to Plymouth, but it would also improve travel between Exeter and Plymouth by providing a direct service between Exeter Central and Plymouth. Likewise is likely to improve commuting options into Exeter from existing stations before the line diverges to head to Okehampton as the existing timetable to Crediton is poor for commuting due to the times it arrives/leaves Exeter.
Whilst the business case which was done does show that it's fairly poor value for money it only looked at running local services and not at running longer distance services over it.