Did a bit of Googling, this might help:
https://www.theguardian.com/news/datablog/2012/may/04/world-top-100-airports
Although it seems to be hard to find all data in a consistent forum indeed. Did you try a source like ICAO?
Yeah, but even there, you'll only get the top 100 or so.
Anyway, with Wikipedia and hard work, here's the final DAT file. Cutoff point is roughly 10% of the prime airport's traffic (or 1000000 handled passengers; except China at 5%).
With the avialable data and cutoff metric used, the list stands at around 300 airports.
Code:
# Exclude airports by entering their ICAO code here, e.g.
# KDFW
# Rule: From busiest to 10% of busiest or cutoff at 1000000 pax
#
# AFRICA
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Africa
FAOR
HECA
FACT
HAAB
GMMN
DAAG
HKJK
DNMM
FALE
DTTA
DNAA
GMMX
FIMP
HEGN
HEBA
HTDA
FMEE
# ARABIA
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_the_Arab_states_of_Persian_Gulf
OMDB
OTHH
OEJN
OMAA
OERK
OOMS
OKBK
OBBI
OMSJ
OEDF
# ASIA
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Asia
VHHH
WSSS
LLBG
# Lower limit: 5% of maximum
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_China
ZBAA
ZSPD
ZGGG
ZUUU
ZGSZ
ZPPP
ZSSS
ZLXY
ZUCK
ZSHC
ZSNJ
ZSAM
ZHCC
ZGHA
ZSQD
ZHHH
ZJHK
ZWWW
ZBTJ
ZJSY
ZYHB
ZUGY
ZYTL
ZYTX
ZSJN
ZGNN
ZLLL
ZSFZ
ZBYN
ZYCC
ZSCN
ZBHH
ZBSJ
ZSNB
ZSWZ
ZGSD
ZSOF
ZLIC
ZGKL
ZPLJ
ZSWX
ZSYT
ZBNY
ZLXN
ZSQZ
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_India
VIDP
VABB
VOBL
VOMM
VECC
VOHS
VOCI
VAAH
VOGO
VAPO
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Indonesia
WIII
WADD
WARR
WAAA
WIMM
WIPP
WARJ
WIDD
WIHH
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Iran
OIII
OIMM
OIIE
OISS
OIBK
OIAW
OIFM
OITT
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Japan
RJTT
RJAA
RJBB
RJFF
RJCC
ROAH
RJOO
RJGG
RJFK
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Malaysia
WMKK
WBKK
WMKP
WBGG
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_the_Philippines
RPLL
RPVM
RPMD
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_South_Korea
RKSI
RKPC
RKSS
RKPK
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Taiwan
RCTP
RCKH
RCSS
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Thailand
VTBS
VTBD
VTSP
VTCC
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Turkey
LTBA
LTFJ
LTAI
LTAC
LTBJ
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Vietnam
VVTS
VVNB
VVDN
VVCR
# EUROPE
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Europe
EGLL
LFPG
EHAM
EDDF
LEMD
LEBL
EGKK
EDDM
LIRF
UUEE
LFPO
UUDD
EIDW
LSZH
EKCH
LEPA
EGCC
ENGM
LPPT
ESSA
EGSS
EBBR
EDDL
LOWW
LIMC
LGAV
EDDT
EFHK
LEMG
UUWW
EDDH
LSGG
ULLI
EGGW
EPWA
LKPR
LEAL
EGPH
LFMN
LHBP
EGBB
EDDB
LROP
EDDK
LIME
EDDS
LPPR
UKBB
LIPZ
LFLL
EGPF
LIML
LFBO
LICC
LFML
BIKF
LPFR
LIRN
LIPE
EGGD
LEIB
LFSB
LSZM
EBCI
LGIR
ESGG
LEVC
LBSF
LGTS
LFBD
# NORTH AMERICA
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Canada
CYYZ
CYVR
CYUL
CYYC
CYEG
CYOW
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Dominican_Republic
MDPC
MDSD
MDST
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Mexico
MMMX
MMUN
MMGL
MMMY
MMTJ
MMSD
MMPR
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_the_United_States
KATL
KLAX
KORD
KDFW
KJFK
KDEN
KSFO
KLAS
KCLT
KSEA
KPHX
KMIA
KMCO
KIAH
KEWR
KMSP
KBOS
KDTW
KPHL
KLGA
KFLL
KBWI
KDCA
KSLC
KMDW
KIAD
KSAN
KHNL
KTPA
KPDX
KDAL
KSTL
KBNA
KHOU
KAUS
KOAK
KMSY
KRDU
KMCI
KSJC
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_the_Caribbean
TJSJ
MKJS
MUHA
MYNN
TTPP
TNCA
TFFR
TBPB
TFFF
TNCC
TNCM
MKJP
MTPP
TIST
SKSP
MUVR
MWCR
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Central_America
MPTO
MROC
MSLP
MGGT
MNMG
MRLB
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_South_America
SBGR
SKBO
SBSP
SBGL
SBBR
SCEL
SPJC
SPIM
SVMI
SBCF
SABE
SBKP
SBRJ
SBSV
SAEZ
SBPA
SBRF
SBCT
SBFZ
SEQM
SKRG
# OCEANIA
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_Australia
YSSY
YMML
YBBN
YPPH
YPAD
# https://en.wikipedia.org/wiki/List_of_the_busiest_airports_in_New_Zealand
NZAA
NZCH
NZWN
Curious what Traffic Database Builder will make of it.
Splitting the large GA aircraft from the small GA aircraft alone produced a twice bigger traffic.bgl file for the large ones.