Where's the best spot to see the construction progress?
The official IKNOW visit route takes you to Plaza Seremoni Barat, from where you can see Istana Garuda, Masjid Negara, Plaza Seremoni, and Taman Kusuma Bangsa, the areas that are most developed and actually designed to be visited.
Because visits run on a fixed route rather than you choosing where to go, the honest answer is: the best spot is whichever one's actually part of the official route. The electric bus stops at Plaza Seremoni Barat, from where you can walk to see Istana Garuda and Masjid Negara, explore Plaza Seremoni to the east, and Taman Kusuma Bangsa to the west.
Why not somewhere else
Areas outside this route, including the legislative and judicial complex that's still under construction, aren't part of general visits yet, so the best current views really are limited to what's visible from the official route.
This will likely change
As construction continues, including things like a glamping area, a forest-canopy suspension bridge, and a cultural center that have started appearing in OIKN's own plans, the list of visitable spots will probably grow. The IKNOW app will be where that gets updated first, not this page.
Don't expect to pick your own route on the day. Enjoy what's visible from Plaza Seremoni Barat, it's genuinely designed as the main vantage point for now.