Can't completely ignore gimmicks and figures since it's part of the experience. Anyway, for me (limiting to three bullet points so I don't rant/gush too much):
1. Superchargers (tied with SF for now)
Pros:
Driving sections have a lot of variety from just two additional purchases
"Knock" combat style makes it satisfying to play and makes melee characters possible to use in harder difficulties
Block puzzles are more than just "push block until you can't anymore" and change with difficulty (supposedly? It asked me if I wanted to change difficulty to an easier puzzle on Normal when I was making it more complicated than it was)
Cons:
No one's ever online, even on Wii U
Loading during cutscenes making them unskippable
1. Swap Force (tied with SC for now)
Pros:
Elemental and Swap Zones have plenty of gameplay variety
"Knock" combat style makes it satisfying to play and makes melee characters possible to use in harder difficulties
Elemental Zones give older characters value, and reason to play as them
Cons:
Loading during cutscenes making them unskippable
You have to listen to Rufus every time you start your completed file
3. Spyro's Adventure
Pros:
Skylander gibberish giving each a distinct personality and feel
Areas and puzzles that can be approached differently with different Skylanders (Flying over ice, sniping thieves, water/lava requiring specific characters to traverse, etc.)
Kaos, who breaks the fourth wall and feels like an equal with his dark Skylanders
Cons:
Combat's nothing to write home about
PS3 and Xbox 360 are earlier builds and thus are partially incomplete
4. Trap Team
Pros:
The villains have fun personalities
You can play as Kaos
Minis are adorable
Cons:
Glitchiest Skylanders game
Traptanium Elemental Gates are the most arbitrary paywall ever
First game to give old Skylanders nothing to unlock
5. Giants
Pros:
Autogyro Adventure
Machine Ghost
No additional purchases required if you already have all 8 elements from SSA
Cons:
Nothing significantly new added from SSA's start
Takes away fun features of SSA
Nightmare mode is next-to-impossible using the titular Giants