In the English Vocabulary Profile, ‘one’ is listed as a determiner at A2 = to refer to a particular person or thing in a group
This is one of the most important documents in history.
In this context, “one” is used to indicate a particular but unspecified example of something, in this case, a document. It’s part of the phrase “one of the…” which is often followed by a superlative adjective and a plural noun, as in “one of the most important documents.” So, it’s determining or limiting the noun that follows it, which is a key function of determiners in English grammar.
An iWeb search:
* * * one of
1 . THIS IS ONE OF 50763
2 THIS IS ONE OF 38872
3 . IT IS ONE OF 34196
4 . ” ONE OF 23960
5 . IT ‘S ONE OF 22692
6 , WHICH IS ONE OF 20904
7 EACH AND EVERY ONE OF 18333
8 , THIS IS ONE OF 18159
9 . IT WAS ONE OF 17056
10 . ” ONE OF 17032
11 , IT IS ONE OF 12608
12 . HE WAS ONE OF 10971
13 , MAKING IT ONE OF 10953
14 CONSIDERED TO BE ONE OF 10461
15 IF YOU ARE ONE OF 9952
16 , IT ‘S ONE OF 9136
17 . HE IS ONE OF 8720
18 , AND IS ONE OF 8343
19 AND THIS IS ONE OF 7852
20 HAS TO BE ONE OF 7693
21 . AS ONE OF 7637
22 IT IS ALSO ONE OF 6987
23 . THIS WAS ONE OF 6763
24 IF YOU ‘RE ONE OF 6284
25 THIS IS JUST ONE OF 6190
26 ) . ONE OF 6158
27 GOING TO BE ONE OF 6040
28 ” THIS IS ONE OF 5990
29 , IT WAS ONE OF 5801
30 . IN ONE OF 5670
31 . THAT ‘S ONE OF 5663
32 AND IT IS ONE OF 5528
33 IN FACT , ONE OF 5503
34 AND IT ‘S ONE OF 4918
35 IT IS ONE OF 4860
36 ” IT ‘S ONE OF 4815
37 HAS ALWAYS BEEN ONE OF 4712
38 IT ‘S ONE OF 4684
39 IT ‘S ALSO ONE OF 4626
40 THAT THIS IS ONE OF 4626
41 IS REGARDED AS ONE OF 4605
42 , WHO WAS ONE OF 4506
43 AS WELL AS ONE OF 4402
44 AND IT WAS ONE OF 4280
45 FOR EXAMPLE , ONE OF 4271
46 OUT TO BE ONE OF 4228
47 , HE WAS ONE OF 4138
48 CONTINUES TO BE ONE OF 4100
49 . HE ‘S ONE OF 4009
50 THIS WAS ONE OF 3985
one of * * *
1 THE MOST IMPORTANT 110018
2 THE MOST POPULAR 80798
3 THE WORLD’S 76426
4 THE MOST COMMON 45080
5 THE BEST WAYS 39919
6 THE THINGS THAT 35290
7 THE MOST BEAUTIFUL 23929
8 THE BEST THINGS 23488
9 THE MOST FAMOUS 22063
10 THE MOST POWERFUL 21537
11 THE THINGS I 21289
12 THE FIRST THINGS 21046
13 THE MAIN REASONS 18490
14 THE MOST SUCCESSFUL 17967
15 THE REASONS WHY 17660
16 THEM. 17423
17 THE NATION’S 16942
18 THE FOLLOWING: 16665
19 THE MOST EXCITING 16301
20 THE MOST EFFECTIVE 15937
21 THE FASTEST GROWING 14574
22 THE COUNTRY ‘S 14221
23 THE BEST IN 13850
24 THE REASONS I 13030
25 THE MOST INTERESTING 12744
26 THE FIRST TO 12619
27 THE MOST DIFFICULT 12480
28 THE BEST PLACES 12146
29 THE MOST INFLUENTIAL 12001
30 THE UK ‘S 11565
31 THE MOST SIGNIFICANT 11392
32 MY FAVORITE THINGS 10737
33 THE HIGHLIGHTS OF 10276
34 THE LARGEST AND 10059
35 THE CITY’S 9937
36 THOSE THINGS THAT 9843
37 A NUMBER OF 9782
38 THE EASIEST WAYS 9779
39 THOSE PEOPLE WHO 9018
40 THE WORLDS MOST 8981
41 THE BIGGEST CHALLENGES 8071
42 THE MOST ICONIC 8026
43 THE REASONS THAT 7975
44 THE THINGS WE 7801
45 THE GREAT THINGS 7667
46 THE MOST UNIQUE 7447
47 THE FOUNDERS OF 7357
48 THE BENEFITS OF 7267
49 THE VERY FEW 6938
50 OUR MOST POPULAR 6592