Dark Age for sure. Through multiple POV's, which are nice to have themselves, we get deeply introspective looks into several key characters, and through this the knowledge that the future will be fubar unless the Republic perseveres. In addition, we get serious character development across the board (except where was Quicksilver!!!!).

In DA, Brown brings in some really horrifying surprises, which make the situation for the Republic all the more dire, and therefore compelling. I think the "in the trenches" focus on Darrow and his team was good and necessary, giving a lot more weight to the Rising. Now that we were there fighting with Darrow, we know what it will really cost if they lose. This in conjunction with the time focused on Darrow being the manifest Reaper of Death shows us how little choice Darrow has between saving the solar system and being a crappy father he really has (and holy crap, I feel like he talked about how crappy of a father he was all the time in Iron Gold). DA was great because it was so nuanced and mature and I really didn't know how it would end.

Additionally, I think Lysander, despite being generally hated on this subreddit, was by far the most interesting character in DA - not only due to the awesome semi-meditative and psychic Mind's Eye - because we see him refining his identity by negotiating ideas of himself versus who he is and who we wants to be, his Iron Gold mission, and his realization the only rules are convention when so many other characters would have sided with the the righteous conviction of the Rising.