Alfa Romeo GTA. Either the 147 or 156. I owned mine for 10 years from 2007 to 2017, and it only lost about £6k of value in that time - and even that only because I knowingly sold it on the cheap to avoid the hassle of fixing an aircon issue first. I sold at £5,500.
In the 5 years since I sold it, it has only gone up in value. Even if the new owner hasn't cossetted it, it'll likely command around £10k now, and they're continuing to go up.
The one issue with that option though - parts availability isn't necessarily great across the board.
The C63 I replaced it with might be a good shout too. I bought at around £30k, and it's probably worth closer to £20k now. But the values seem to have stayed fairly static or recovered a little over the last couple of years.