For me its generally jets that I collect....( HUGE fast jets fan!!)
So I've collected each of the aircraft type I really like, with one sample of each, but this might be extended to four or five of the same type too!
I'm big on my campaigns and appreciate significant eras in military aviation history. I will also incorporate some dioramas in my collection to spice it up a bit!!
Here are some of my shared examples:
USAF Icelandic 'Zulu Alert'
Vietnam War
Cold War
Iran/Iraq War
Gulf War (1&2)
Yom Kippur
Rhodesian Bush War ( converted RAF Hunter to a Rhodesian one)
South African Border War
OIF/OEF