Wife going to concert at start of June.

What is best tube?

Or is it cab only territory?

http://www.theo2.co.uk/location/transport.html#tube

The one next door to North Greenwich tube station?

Docklands DLR, or Jubilee There is a station about five minutes walk away, really easy to get to and from, even when busy. I’ve seen Elbow there twice, the last time it was sold out, 18,000 or so, but it still only took fifteen-twenty minutes to get away. The actual stop on Jubilee is North Greenwich for The O2, so you can get the DLR to Canary Wharf, which give you good views of the Docklands area from above, then change to Jubilee to The O2, or else pick up Jubilee at London Bridge on the Northern Line.
Much, much easier than fannying around with taxis, for which you’d probably have to sell a body part, or family member to pay the fare…

The nearest DLR stations are at Greenwich (10 minute bus ride), Canary Wharf (Jubilee from North Greenwich to Canary Wharf and 5 minute walk) or you could get the cable car to Royal Victoria Dock DLR. You could walk 20 minutes to Maze Hill and get a train into London Bridge possibly.
Depends where you’re coming from/ returning to afterwards.
But as someone who goes to North Greenwich every weekday I’d suggest just getting the Jubilee Line either to Stratford or central London.