It's situational, you understand...like the best legacies for doing what?
However, since you seem to be asking PK related questions I'll go that direction. Hope that's okay.
One of the things everyone should do is use the amazing PK stats tool. Search the top 20 highest KpH warriors and look for commonality. While this doesn't yield the truest answer, necessarily, it is very useful information. Sometimes you'll do this to follow the tried and true trends, other times you'll do it to buck them.
Legacies that everyone takes for good reason:
1. Striking the Shadows Footfall
This is the best legacy in the game. Back when no one used to take it Nep was up in immland wondering WTF. His (nearly) exact quote to the rest of us was, "It does more for you than any other legacy. At full effect it's like have four legacies for the price of one."
2. Greeting the Avalanche
Command denial is the most effective way to kill people. It's just plain simple and requires very little skill. If you can bash and out damage them then bash bash bash. See also: bash; bash;bash flurry!
(Always of note to me the ((arguably)) two best legacies in the game are mutually exclusive. Those that benefit from STSF don't benefit from GTA, typically.)
Legacies that fit my play style really well:
1. Riddle of Resilience.
Probably my personal favorite legacy. Typically when you're a powerhouse (from years of playing and knowing what you're doing) people who don't flee will try to steal stats. This is especially true if you can kill them through command denial or burst damage. As I tend to build my warriors for these two things it's common that I benefit from covering stats. Riddle + good stat covering armor makes it impossible to lower stats below racial max. That destroys the most commonly deadly warrior build in the game - the dagger/stsf elf and all its variants. Riddle is particularly strong when people don't know you have it and keep trying to kill you with a strategy that can never win.
2. Calming the Shadow's Tempest
Second to riddle only because there's more warriors than casters. Clearing the air on what this legacy actually does, as there's a lot of misinformation, what it -actually- does is give you a second save vs. spell throw.
This means if you have low saves the legacy won't do a ton for you. If it's easy to miss one saving throw it's easy to miss two. If you have high saves though, man oh man, you'll literally save 99% of every spell thrown at you.
I'm kind of foggy on the details now, but you can also make a half of a saving throw. This means the spell hits you, but not as hard. It's easy to see against invoker spells. A complete failure of saves and you get hit for an unspeakable. A half and maybe you just eat an obliterate. A success and you take a mangle. You dig? Even with bad save vs. spell you'll almost always make the half save if you have calming.
Again, this works best when people don't know you have it. This legacy plus lash and you can defeat any shaman or mage. This legacy + spellbane? JesusGod. Ragers who actually want to be the bane of magic probably shouldn't take anything else.
A final thought on this legacy - having done it and forsaken thoughts probably a half dozen times each I can safely say I'd rather have Calming than Forsaken against shaman and druids and stuff. Saving throw > level of incoming commune + it works on more than communes.
Other legacies:
Any legacy can make you successful if it's paired with the right stuff. Many legacies make excellent one trick ponies that are sneaky and fun. I think some of your questions involved specific building so we can get into it there, but I've seen very bad ass builds around almost any legacy.