I have lots of iMacs, all had upgrades:
Late 2012 27" iMac-I got the BTO options for the i7, 680MX 2GB, and 1TB Fusion Drive. I upgraded the RAM myself to 24GB. It is my main Mac that I use, and I may upgrade the internal drives, either just replace the HDD to SSD, or maybe both drives and run them in a RAID 0. I think this should get me about 1000MBps read speeds on the boot drive.
Mid 2011 27" iMac-This was a dumpster find that a friend found and gave to me to see if I could fix it. It had a corrupt HDD and the GPU was fried. I end up doing a GPU bake in the oven to fix it and this was a top of the line BTO iMac from Apple, had every possible upgrade. I also replaced the very hot-running HDD with a SSD. I might add a second SSD using the unused SATA3 port and run a RAID0.
Mid 2007 20" iMac-I picked this one up on craigslist for really cheap in 2010. I have since maxed the RAM and replaced the HDD with a SSD, and gave it to my parents to use. I have not plans to do anything else to it.
Late 2003 17" iMac G4-Got this one in 2004 or 2005 from a police auction, it was missing the internal drive. I max the the internal RAM and user replaceable RAM and put a 320GB HDD in it. It is my favorite Apple design, so I would like to mess around with it more one day. I might put a SSD in it.
1998 iMac G3-Got this in the early 2000's for $60 from an ad in the local paper. I replaced the HDD, upgraded the vRAM, and maybe the RAM. I think I also added an AirPort Card, but can't remember. I might put a SSD in it one day.
I would say the Mid 2011 is one of the best iMacs due to the many possibilities and combinations of things to do. It has 3 SATA ports, a non-soldered GPU, a couple GPU plug & play options, is really easy to work on, and lots of parts out there for it.
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I had a 2011, but the GPU burned up.
If you still have it, consider doing a GPU bake on it. It fixed my Mid 2011 iMac with the still very expensive 2GB GPU.