I've played all the Bionicle games. Most people would probably agree Mata Nui Online is the best, but it's a point and click. As far as the action games go the best is this BIONICLE on GBA. Yeah the graphics and sound are both horrible, but it's actually a pretty decently functioning isometric platformer with interesting levels and some decent puzzles. The Toa's different abilities are actually interesting to play with (Gali water levels with slowed movement and more jump height, Lewa's gliding, etc). Kopaka's making ice blocks to jump on and slide on rails is by far the best level.
The combat is very lame with just auto aiming projectiles but it's still more interesting than the 2D platformers or the other auto aim shooters that don't really have movement mechanics.
As a kid, BIONICLE GBA took me a long time to figure out and beat with unlimited lives. It's actually a very fast game to breeze through if you know what to do, wheras the 2D GBA platformers feel like a bit more of a chore with slow movement and long but boring sections.
Unfortunately there's no save battery, instead there's a password system. But the horrible font has the same symbols for different characters, making it useless. I just left my GBASP plugged in the whole time. You also have limited lives for the whole game, so if you can't save, it's hard mode, but I honestly didn't mind that.
Seriously, I've gone back to play this ugly kusoge more often than any other Bionicle game.
These days you can use save states on an emulator, but at least just save at the level selection screen, don't save scum the levels.
The ps2/gc/xbox/pc version is of course decent for one playthrough, but I didn't really want to go back to it. I think the artwork is fairly decent but it's boring. That was my first game I ever bought with my own money, and I was slightly disappointed with it.