If you like the feeling of cars headed straight for you, I say H-a. That said, position G means you see the cars a bit longer as that is much a much longer straight than the one facing H-a (although in that case, the cars are headed away from you).
Why not choose E-b? You would have a good view of the cars throug the whole complex there (from the corner at H-b until the hairpins at the end of that) and you get to see what’s happening at the hairpin between the small and the long straight as well. With binoculars you might get some good views on what’s going on at the other side of the circuit as well.
Or why not D, it looks like there might actually be some overtaking in the hairpins there (if there will be a chance of that). But the H-a does seem to make sense, you get to see a large area of the track as well.