From a gameplay perspective, IXS is the best. The minigames are decently varied despite them mostly revolving around driving. Exploration is encouraged through brickimal bricks and character cards scattered about the island. The physics aren't ridiculous compared to LI2. I plan on playing it again.
LI2 is a hot mess, but it does have a "plot" and its minigames are extremely varied. It would be a decent game if the loading times weren't 10% of the total gameplay and if its various issues were ironed out. But that would make it IXS. LI2 is to LEGO games as Zelda II is to the Legend of Zelda series. It wasn't really great, but no one knew where the series would go this early on, so they tried different things.
LI1 isn't a game. It's a playset where you walk around, watch people jump out of nowhere to scream at you (WOOOHOHO MY GOODNESS GRACIOUS), turn flowers to cups, and occasionally build a car or deliver a pizza. Two of the minigames are races on unique tracks, while three of them are the exact same thing with a different route (drive from A to B, watch cutscene as Robb said). These games barely change for each character. The pizza routes change but that's it, I think. Only Pepper has a minigame unique to him and it's kinda bad. This wasn't a problem when the game came out because I was 7 years old, which is within the target demographic. At that time, I remember the helicopter portion of the Brickster chase being a load of garbage and I still think that today, if that's helpful.
Despite that, LI1 has a lot going for it. It's the first LEGO game, and it has really good voice acting for a late-90's PC game. The graphics are decent for the time, and the stereo effects are pretty neat. It also has lots of classic music. I think IXS has better music, but LI1's is more memorable. We don't talk about LI2's music.