adding alot of extra information and complexity, but since there's alot of choice at 70+ now in weapons, it's probably justified here: (weapon names in bold, discussion of each weapon below).
pre 66 - pretty much
soboro sukehiro > all * (see footnote on soboro)
66-72 - now things get a bit complicated so we'll discuss it by sub and role.
- Sam/thf: Pretty much should get a good damage weapon. you're doing this for the high hit SA Gekko right? well. a 40 damage gkt isn't going to get you there, sorry. use an onikiri, zanbato(+1), kazaridachi(+1), or messhikirimaru
- Sam/war: bit of a choice here. Gekko is gonna suck with a soboro, but you'll be doing twice as many - if you're not Skillchaining or the blm is bursting it dead anyway, then soboro is best. otherwise consider one of the above 6 swords.
- Sam/rng: Style choice: High Damage bow + soboro, or speed bow + one of the above six weapons. good high damage bows are: war bow +1(longbow) rapid bow+1(shortbow) eventually you'll want to go to a Shigeto Bow(+1)
in addition to the listed weapons, the following EX weapons should also be considered:
kumokirimaru,
Kamewari,
hotarumaru,
Imperial bow.
at 72 the
shinsoku and
hagun are both available. Hagun is generally accepted to be better than shinsoku (I can run the actual weaponskill formula for yuki/gekko/kasha if necessary in a followup post). - however.. what isn't as clear is if hagun > soboro: based on some tests I've witnessed involving lots of parsing (admittedly at 75) soboro and hagun have similar overall damage (ws+dot) with hagun significantly better per hit, especially per ws. for sam/war and sam/thf hagun is generally accepted as the gkt to use from here going forward. (although for sam/war it's basically a wash if you aren't Skillchaining).
at 75 you will generally also carry a gkt used specifically for 300TP ws: I list them below, more or less in alphabetical order:
butachi(+1),
kamewari(ex),
kumokirimaru(ex),
onimaru,
shinsoku,
ushikirimaru(ex).
excluded
amanomurakumo and
futsuno mitama from discussion for reasons that should be apparent.
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(each sword discussed above)
butachi(+1): good damage, good dot, highest straight damage non-ex gkt.
hagun: martial bonus is very very strong- increases multiplier by a further .5 (33% increase in damage multiplier) for gekko/kasha/yuki
hotarumaru: best dps rating outside of relic. potential for very strong elemental ws's if you use them regularly (I haven't had a chance to test the bonus myself)
kamewari: free 5 acc. great crit bonus vs. weapons/pots/etc. ideal for sky and moon camps. not so hot in AU.
kazaridachi(+1): good old fashioned straight damage in the sub 70 range.
kumokirimaru: 80+ damage gkt (top 'tier'), zanshin bonus. +crit. (sick +crit vs. vermin: great for waomora/crawler etc.)
messhikirimaru: kazaridachi+arcana killer enchantment.
onikiri: good vs. demon, decent damage for <LV70 gkt.
onimaru: latent damage is highest of the available gkts, ex or otherwise. active on WS and <100tp. also includes 18 attack and 5 accuracy.
soboro sukehiro: basically a ridill you can use hasso with: you shouldn't need me to tell you how good ridill is.
shinsoku: 80 damage, 5 str. best +str on a gkt in the game. doesn't do as much on ws as hagun will until the 250+ TP range; except against very high def mobs.
ushikirimaru: highest damage ex gkt that has actually dropped. 3rd highest damage gkt in the game (counting relic as one gkt and excluding latents).
zanbato(+1): see comment about kazaridachi. slightly easier to make a 6 hit TP setup with.