There is none. There is good ammo for different situations:
Shredder Rounds VII (notice, they only go up to VII!): Best against organics, does massive damage to anything not robotic (geth, turrents, husks)
Tungsten Rounds VII (again, only up to VII): Best against synthetics
Inferno rounds can be helpful on enemies that hide a lot, as they do damage while they are hiding once you make hits.
Snowblinds can make a target inaccurate and slows their firing... kinda useless in my opinion.
High Explosive rounds are fun, but require lots of heat dampening (Frictionless materials) to be useful... or you can use Rail Extensions to make a one-shot mega-hit (with a maxed shotgun, can be an almost one-shot kill, but overheats so you need to do it only as secondary).
Sledgehammer is fun for shotguns and sniper rifles. This can cause enemies to fall over when hit. Great for a kamikaze style with a shotgunner who has monster melee bonuses to really hit people when they are down
Polonium rounds are essential when fighting Krogan, as it prevents their regeneration.