|December 12th, 2023
Within the p-series there have been tiers, and within each tier Yamaha had many years when they'd make new versions that were very minor changes. Currently, the p-series tiers are:
Higher tiers cost more, but generally give you more or better features: physical response ("action") closer to a real piano, more voices, more pedals, more realistic voices, better interface for choosing voices, more simultaneous notes (polyphony) etc. Here's my attempt to make sense of the historical lineup. I've marked them with their release year, and in cases where they have a fancier action than GHS I've marked that as well.
Go back to the 90s and there's also the p-100, p-150, p-200, p-300, and p-500, which are all old enough that it's harder to get information on them, and you're much less likely to run into them.
Personally I'm happy with any of these; the features distinguishing the fancier models aren't ones I need. I like the built-in piano sounds (Piano 2 and Electric Piano 2 on my p-85) and at least from the p-70 onwards, use external midi for other voices, don't use the pedals, don't need more polyphony than I have fingers, use external midi for most voices, can handle either USB or 5-pin MIDI, and I like how light the GHS action is.