On the surface, it seemed like it would be a tough decision. The first option has some great albums in Fly By Night, 2112, AFTK, Hemispheres, Vapor Trails, and Clockwork Angels. When matched against the 80s and 90s output, though, for me, there's really no other choice. Permanent Waves, Moving Pictures, Power Windows, and Grace Under Pressure are all consistently great, and this period also contains my sentimental favorite in Presto (the first Rush album that really clicked with me to love it, back in high school around '96-97ish). Hold Your Fire is still good in spite of its overall mellower nature. And even my lesser liked albums in Signals and Roll the Bones are still miles above those in the other eras (S/T, Caress of Steel, and Snakes and Arrows).
So 80s and 90s Rush it is.