best overall legion i'd say was death guard rules wise at the moment. thats based on their legion rules, wargear, primarch, units, and rite. they don't have the best, but their overall consistency is probably the highest. the night lords are probably a good second with a strong argument for best.
iron hands have the best legiones astartes rule gaming wise. (-1str to shooting attacks against them) although the DG's flamer weapons have shred is pretty nasty, (personally won more than a few games becuase of those re-rolls during overwatch.) however I feel that the word bearers have the weakest. with it being more restrictive and 'plain' when considered next to other legions.
however I think that the world eaters have the best rite of war. Which gives the entire army hatred, which when added to their 'legio astartes' rules from massarce, gives the entire army rage and any unit that wipes out or makes a enemy unit flee furious charge, makes even a basic unit a complete combat monster.
night lords come in a very close second, theirs gives night fighting on turn one if they want it, and they can carry it over on to turns 2+3 on dice rolls, and considering the entire army has the night fighting USR, thats a complete one sided bonus.
I'd also consider the emperor's children rite to be the weakest, as it's only notable addition is to allow you to outflank 1-3 elite/fast attack choices on a pregame fixed turn. although its good, and could be game winning, the restrictions placed upon it means IMHO its not worth it.
think the Death Guard and salamanders are tied for best Legion exclusive units. Deathshroud/Grave wardens Vs Pyroclast/Firedrakes. the other legions either have one great one and one so so unit, but the DG and Salamanders have the best 'averaged' units. IMO the Iron Hands probably have the weakest set of legion exclusive units. the gorgon terminators cause blind to all nearby models (which could easily effect friendly units more than enemy ones) and the immortals are just breachers with a few extras, but they still have the inherent issues of breacher squads (high cost, circumstantial defences).
couldn't say which legion has the best/worst characters as each character has their own 'thing' and their all priced differently (some are praetor level, while others are centurion level) although typhon does stand out from the bunch in massacre alongside sevatar who is pretty nasty for his points cost.
the best/worst primarchs rules wise is even tougher. I think curze has some interesting and potentially devastating buffs (giving his entire army fear, and units with it already get a -1 to ld tests for fear) specially when combined with the NL rite and legiones astartes rules. vulkan could also be a contender being nearly as tough as mortarion but with more buff and more killy. but horus remains the best all rounder.