Three clauses of Magna Carta 1297 still have force of law in the United Kingdom
"1. FIRST, We have granted to God, and by this our present Charter have confirmed, for Us and our Heirs for ever, that the Church of England shall be free, and shall have all her whole Rights and Liberties inviolable. We have granted also, and given to all the Freemen of our Realm, for Us and our Heirs for ever, these Liberties under-written, to have and to hold to them and their Heirs, of Us and our Heirs for ever.
9. THE City of London shall have all the old Liberties and Customs which it hath been used to have. Moreover We will and grant, that all other Cities, Boroughs, Towns, and the Barons of the Five Ports, as with all other Ports, shall have all their Liberties and free Customs.
29. NO Freeman shall be taken or imprisoned, or be disseised of his Freehold, or Liberties, or free Customs, or be outlawed, or exiled, or any other wise destroyed; nor will We not pass upon him, nor condemn him, but by lawful judgment of his Peers, or by the Law of the land. We will sell to no man, we will not deny or defer to any man either Justice or Right."
Clause 29 also remains part of the legal system of New Zealand (and possibly other Commonwealth states).
Ireland has England beat on this one. The oldest piece of legislation on the books (since 2007, when most every law from before 1922 was repealed) is the Fairs Act 1204. From what I can make out from Dáil debates and so on, it was an act setting up fairs and regulating castles in Irish cities.
There are also a bunch of other thirteenth century laws still in force. The oldest one that's readily available online is the Sheriffs Act 1293, which is about judicial practice and landholding. (And, although legal history is not my strong point, looks like a very important piece of legislation.)
EDIT: Punctuation.
The 1491 wedding contract uniting France and Brittany also contained a close forbidding France from placing tolls on any road or bridges in Brittany. As a result there is no freeway (130km/h speed limit, contains tolls) in Brittany to this day and only expressways (max 110km/h, no tolls).
In terms of international law, what I commonly see cited as the oldest treaty still in force is the Anglo-Portugese alliance, which dates to 1386 and was an extension of an older treaty cited in 1373. I'd be curious to know if my understanding of this is correct or if there are other legally binding international treaties.
As /u/phoenixbasileus (nice username btw) has pointed out, this honour probably goes to the Magna Carta. The next best one I can think of is the constitution of San Marino dating from 1600, but since this is a really interesting question I’ll try to remember if there’s anything earlier than the Magna Carta.
EDIT: There’s probably something older than the Magna Carta in internal governing laws of the Catholic Church, a papal decree or similar. Maybe some experts on Catholicism could give us more info on that.
This submission has been removed because it violates the rule on poll-type questions. These poll-type questions do not lend themselves to answers with a firm foundation in sources and research, and the resulting threads usually turn into monsters with enormous speculation and little focussed discussion. “Most”, “least”, "best", "oldest", and "worst" questions usually lead to vague, subjective, and speculative answers.
Although there are a few good answers in here, most of them are trivia responses and many violate the 20-year rule.
How far back do land ownership records go? I don't know how it works elsewhere (really: anywhere) but we buy title insurance incase someone finds a problem with the transfer of property, no matter how far back.
Ok, in the US, I've no idea how come anybody can own the land once we started letting the Native Americans be human beings in front of the law. So for sure our title records only go back to 1500s or so.
But people all over the world have owned land (whatever that means) for a long time. Aren't those transfer of title documents still 'in force'?