In terms of lasting peace and stability, the Control ending with a Paragon Shepard would provide the most benefit to the most individuals. With the near infinite knowledge and power of the Reapers, combined with her own intellect and compassion, Control Shepard would guide the galaxy into a era of prosperity and stability. If the krogan or rachni get out of control, the full force of the Reaper fleet would force-er, gently guide them back into line. If a new AI threat like the geth arise, Shepard could negotiate peace. Wars would be a thing of the past. With the Reaper fleets patrolling the spaceways, crime would nearly vanish. With the capability to create new mass relays, resource shortages would no longer be a problem.
Oddly, Destroy going the "happy ending" route for the plotlines would likely leave the galaxy in the worst state. Krogan would breed out of control - even with Wrex and Eve, there's nothing even close to a guarantee they would be able to change the essential nature of their species. The rachni are so completely alien, it's inconceivable there wouldn't be at least skirmishes with other species. After having a taste of geth assistance for a few weeks, the quarians would scramble to recreate them...and the temptation to make them more subservient might be overwhelming. Without a single threat to unify them, the galaxy might experience a few years or even decades of peace, followed by a return to the status quo.
Synthesis has too many outstanding questions to adequately say what will happen. Best case scenario, millennia of wisdom from countless civilizations provided by the Reapers combined with a new feeling of unity and interspecies understanding provides peace without a benevolent overlord. Worst case, you've got all the problems of destroy but with the unknown variables of the freed Reapers in the mix.