
My first experience was with the SRH-440's. Thought they were the best ever.

ATH M50s. Used them happily for two years, but when the earpads disintegrated on me I started looking for upgrades, which led to a slippery slope...
Senn. HD598s
Answering to this question: the Pioneer SE-L40. Not mine, but a friend owned them. After that day, i started my own headphone collection.
I started with the Ultrasone proline 750
as of now i only have one headphone, ma900's
HD202, not sure if that counts so i will state the HD558 too
AKG K271 MKII. I have now upgraded to a pair of K550s
For me Sony MDR-7506. I still have them and use them almost daily.
Elega dual driver CH-232 which I came here to ID after years of ownership and periodic listening, then first real set after looking around in here ( after the Nu-Force NE-7M iem ) was the 600 ohm AKG 240M locally. That's how the terrible curse started for me.
While I appreciate good sound I in no way consider myself an audiophile I acquired my first proper set of headphones just yesterday. A shiny new pair of AKG Q701s. Prior to that my only set of decent cans were some Seinheiser HD280s.

-Kit
My first real ones where the Sennheiser HD598 and I love them to pieces. From comfort to sound to looks, everything is great
Ultrasone HFI780 for me 
Sony MDR-V6.
Found em in a bin in my basement with my grandpa's old stuff.