iPhone 3GS, 4, 5, 6.
I stick with a 2 year cycle, the only exception being the 3GS to 4 as it came free with a bundle from Telstra. My next handset will be the iPhone 7, or whatever it’s called.
For those interested, in reverse order had a Sony Ericsson C905, Sony EricssonK800i, Nokia N70 and a variety of other Nokia/SE phones prior.
I’m on this ‘S’ cycle. I upgrade every two years.
Started with the 3GS, then 4S, 5S and now 6S.
I started with a 3GS, which I absolutely loved, but passed on both 4’s because I didn’t like the square vibe. The 5 really did it for me though and I ordered one the minute they became available. Still have it & still like it. Am trying to get hold of a 5S now because none of the 6 series are my thing. Hoping next year will bring us an updated smaller screen phone.
I buy the latest iPhone every year - it’s a treat to myself, hah. I could probably get along with an iPhone 4 fine and only upgrade when Apple stops release updates, but I like new shiny:)
Best iPhone imho is the iPhone 5c. I loved the plastic case. The iPhone 4/4’s glass back was weird and the iPhone 6/6s is a slippery bar of soap in comparison!
Just the one.
I had an iPhone 4 for almost 3 years, given to me by the director of the company I work for. Gave it up when he asked for it back - to give to his father!
It was getting pretty long in the tooth by the end though, and each time Apple released a new phone it felt just that little bit older. I suppose that’s what Apple want iPhone owners to feel though, otherwise there would be no upgrades.
And when I handed it back I arranged for it to be replaced by a BlackBerry classic, 'cos I love phones with buttons and my other phone is an 8 year old BlackBerry Pearl 8100, still going strong…
I have got the 4, 5, 5S, 6, 6S all on launch day because I couldn’t help myself… Or I should blame @decryption thanks to his iPhone buy guides teaching me about Telstra’s re contracting + device buyout.
I collect Apple products so i have all of them from the 2G to the 6S with the exception of the 5 (sold at the time of the 5S) and the Plus phones.
On that note if anyone has a 1st Gen iPhone 2G box they want to sell cheap (Not the phone… just the box) send me an inbox. 
Oh and i really wanted a 3G /3Gs back in the day but i was stuck on a Hiptop plan. Sad times
I still miss my white 3GS. So nice to hold! I imaging the 5C is like that a bit. If they released a iPhone 6 bits in 5C case I’d be allover that! I use a 5S currently, as I love the size, but I couldn’t go without Touch ID these days.
There’s one for sale in the trading thread now…
Which I don’t have access to yet. 
Hmm, lets see, 3G,3GS,4S (x6),5C,6S+. trying to hold off the next update as long as possible. really enjoying the 6S+ I had to go through quite an ordeal to get it. it’s been a good unit so far.
(spotted my thread jacking will fix, Apologies All)
And since then also…
6S+
7 (current phone)
Not as many as some 
2G, 3GS, 4, 4S, 5S, 6S
3GS (2012-2013)
4 (2013-2014)
5 (2014-2016)
SE (2016-Present)
4, 4S, 6 plus, 6S
One - the current model SE.
And since then a 7 Plus and 8 Plus
Like SteveClark, one, an SE a bit over a year old. (It replaced a Nokia 3310.)