Hmm, ok. Of course this is highly personal. I for one am convinced that Navcore versions 9.*** are too heavy for 32 MB / 266 MHz devices (i.e One, One XL, XL without Live or IQ, Go classic, Go 300, 500) but others might say it runs fine.
So here goes:
Go Classic, Go 300, Go 500, One, One XL, XL up to 2008: either 8.562 or 8.417 SE, possibly with Vocalizer TTS. Run in x40light mode with folder \outsourced.xxxx present for memory saving.
Go 700, Go x10: Still 8.562 or 8.417 but also 9.051 if you want better graphics. Run in x40 mode and there is no need for the \outsourced folder. Use Vocalizer TTS or, if you have the 910, use the original LoquendoTTS it came with.
Go x20, Go x30: If you want all options of your device to work, especially EPT from 920/930, go for the original 8.351 and patch it. Otherwise, try any 9.*** version and it will work. 9.051 works nicely on my 920TMC with TMC also working, provided you choose x30 or x40 mode. I have no experience yet with 9.101 SE.
Go x40: Again, if you want all options designed for your Tomtom to work, stick with 8.371 and patch it. Otherwise, 9.051 in x40 mode or possibly 9.101.
Go x50: The original navcore I believe is 9.026 so you can use it and patch it. Otherwise 9.051 or 9.101 in x50 mode.
XXL, XL IQ and XL Live (2009): Use the original navcore for those devices, they are already 9.***. Yesterday someone here complained that his XL crawled to almost a halt when he updated from 8.562 (Original) to 9.053 original, so in some cases it might be best to stick with 8.562...
Start/Ease (version1): 8.544 ME (Mireille Edition) by DarkMireille. Do a Google search for these words.
Start2/Ease2: The original navcore for these devices, or possibly also 8.544 ME or 9.051 ME. Again, google.