The rune longsword is slower, but has a higher max hit by 1, but the rune scimitar is faster but has some unnecessary bonuses and is expensive, so the rune longsword is best.

You can buy a rune longsword at the grand exchange.

A longsword. He says to his wife, "Give me my longsword, ho!" His wife thinks it would be more appropriate for a man of his age to wave around a crutch.

a dragon longsword changes over time but is worth around 99k

The best longsword, according to the runescape wikia, is the chaotic longsword. The chaotic longsword is obtained via the dungeoneering activity.

It costs 200,000 tokens and if completely uncharged will cost 2 million coins or 200,000 coins and 20,000 tokens to recharge to 100%.