Pretty much anywhere... There are no bad spots.
From the top of paddock you can see most of paddock (it dips away so steeply you can't see the middle bit), druids, graham hill bend, surtees, clearways and the whole of the start/finish straight. Probably the best place to watch.
At Druids you can get close, and if they have the inner part of druids open there's great viewing to be had there of druids, graham hill bend and paddock.
From the southbank you can't see quite so much but you can hear everything all around you, you're sort of smack in the middle, its pretty cool
. Then there's clearways and also next to the med centre on the exit of surtees where you get a decent view.
Out on the grand prix circuit, viewing is a lot more restricted because the trees give you a narrow field of view, but you're still very close to the action. You can only get to the inside of the corners too. standing on the Apex of Hawthorn is my favorite, closely followed by in the dip at Dingle Dell.
Generally the best thing to do at Brands is just spend all day walking around and taking in all the views, there are so many great places to watch it'd be silly to spend all day in the same spot.