Well, I can't talk much about the original trilogy, as I've not played much of them, what I can tell you is that they're much more lighthearted in tone in comparison to the Future games.
Gladiator, All-4-One and Full Frontal Assault/Q-Force, while they are considered main series entries, they're as close to spin-offs as the series gets. Gladiator is all about arena challenges inspired by some of the mini-arena challenges from the original trilogy games. All 4 One is a co-op title, and is only really fun when playing with others, by itself, it's decent, but not all that engrossing. FFA/Q-Force uses more traditional gameplay, but is inspired by tower defense games, it's an odd little spin on the series, and I'd try out a demo to figure out whether or not this spin is to one's taste or not.
And now, onto the main Future games, I've played all of 'em barring Tools. Quest for Booty is pretty short, being a PSN downloadable title. It's fun, but obviously lacks "meat," so to speak, you can play it for completeness' sake or for better bearings on the ongoing plot of the Future arc, but I wouldn't call it a must. Crack in Time is my favourite of the series, it's probably the most story heavy, mind you, and the most serious. But, even then, it doesn't lose the characteristic sense of humour, and the weapons are still pretty fun. Finally, there's Nexus, it's smaller Crack in Time and Tools, but larger than Quest, it does feel a bit more linear and constricted at time, but it's pretty much just classic Ratchet and Clank, it ties up the Future series, so if you're in it for the story, you'll probably want this one.