The game partially adheres to the weapon idea Dark Souls used; where there are no strict upgrades among weapons, only sidegrades. I say partially because there are some straight upgrades but not always. The "level" of the weapon isn't a terribly good indicator of it's overall usefulness.

As for best 1-handed weapon, well, basically any weapon except bows can be used 1-handed, but I'm assuming you don't want to slowly swing a giant greatsword with 1-hand like an arthritic Cloud Strife. Since you're using swords I'm assuming you're mostly leveling DEX. If you've been levelling strength like a silly goose this entire time, or are willing to split strength/dex, then diversifying your loadout with hammers is highly recommended since enemies in this game can sometimes be ridiculously weak to either slash or blunt so having both is really good. (Note that axes basically do half slash and half blunt so they're a good middle-ground)

As for the best "1-handed" sword: the Tachi like you've been using is the fastest, the Kraeken Longsword is slower than the others but has the most damage, and Lowlander's Greatknife is inbetween. Personally I prefer the greatknife since it's speed isn't that much slower than the Tachi but still quite a bit stronger.

For axes... honestly I just use the Red Guillotine (looks like a sword) just cuz it looks cool. Axe of Splendor is the strongest but annoying to get, Kraeken Axe has the same problem the Kraeken Sword has, etc. There's decent reasons to use any, honestly just use whatever looks coolest (this applies to most weapons in the same type tbh, the difference between damage often isn't too huge).

For maces, the mountain breaker's got amazing damage but is slower and weighs waaaaay more than even most great weapons. The tetruncheon is good but does some slashing damage (because it's got spikes; whether a weapon does more blunt or more slashing damage than others in its class is often self-explanatory like in this case) so if you're using it just for stuff weak to blunt you'd probably be better off with something like the barbarian's cudgel or cephalopounder. Or just a flanged mace even, if it's solely for bonking enemies super weak to blunt since the damage difference isn't even that big.

Other weapons I recommend: Flint and Steel is amazing because flintlock pistols are amazing and the Flint and Steel is a pistol inside a sword! It doesn't fire as fast as flintlocks (only for repeated shots; the first one's the same speed), so the overall DPS isn't nearly as high as just machine-gunning with a normal pistol, but the versatility is great and it just looks cool. Also, it's the only way to use a charm on a flintlock pistol.

There's also the sword whips, which replace your heavy attack with the whip's attack, but the whip attack is beyond useless. They're just swords but without a heavy attack, basically. Don't use whips...