Whats a good tree or shrub for a barrier?

If you don't want all the light blocked, but just want to create a basic screen, it would be better to have something deciduous that leaves a woody framework [loads of things would suit] or pick something which isn't too dense.
The Cotoneaster mentioned is a good choice for that, although some of the evergreen ones are quite solid.
Anything you plant will need the site well prepped though. It's looks quite congested on the inside, so you'd have to either choose something that will be happy with that, that you can get well established, or remove some of those plants first.

Alternatively, you could just use an actual screen. There are some very nice sculptured ones made of metal/plastic, or you c ould just use trellis. A climber could be added - whether it's a perennial or annuals, to give you some extra colour.