From what I can gather, the New Republic basically ended up being the same as the Pre-Palpatine Republic shown in the Phantom Menace, too caught up in the nitty-gritty politics and not taking action when needed. Hence why they didn’t stop the First Order until it was too late. (Though they did help back the Resistance so they get a little credit there.)
The only sensible answer is the Galactic Republic just based on how long it lasted. In comparison both the Empire and the New Republic barely warmed their seats before it all came crashing down.