Oof, you ask a hard question...
My top favourite would have to be Lego Racers 1, simply because it's stable (unlike LRR), lots of things to do in single-player, and a very fun two-player mode.
However LRR is up there as well for a "casual RTS" of sorts (what even is this type of game called?), as well as Lego Star Wars 2 (while I don't actually have a copy, I have played it with several friends - and also in my orthodontist's waiting room! I do have LSW1, and that was great fun!) for an extremely fun 2-player mode. LR falls down a bit here because I am much better at LR than any of my friends are however it is very fun to, instead of winning, just try and shoot the other player with as many cannonballs as possible
Shockingly high up on the list is CrystAlienConflict, that Flash game for Mars Mission (which is still playable off BioMediaProject), simply because it's a Lego RTS and we need more of those. Please.
Barnacle Bonkle Bionicle Heroes is also quite a nice game, but the canister collection is annoying due to all requiring Vezon to beat the game (there's no way you can collect all canisters on a first run, something I find annoying), and it's not quite up my street as much as LRR does. BH becomes a lot more fun if you never buy any armor / weapon upgrades, and force yourself to not exploit regenerating masks. Now it actually becomes a (slightly) difficult game. Under this setup, the Vezon fight in Thok's Grotto is literally impossible because you have no room to move about. And it doesn't help that the regenerating masks are Nuparu and Hewkii... the two worst characters for hitting Vezon (Hewkii is the worst). Winnable, but you have to abuse the masks that keep popping up.
Alpha Team fails to make the cut because of the level design - there's really only one way to skin a cat in most levels - and also the annoying "oh, if you moved it one stud to the right you'd have won" as well as a slow UI (there's no way to insta-run tests and you have to wait through the "You lose!" animation unless in Puzzle Editor mode).
Much lower down on my list are LR2 and Stunt Rally - both games I completed, but didn't really enjoy the completion of them. SR has a nice level editor which I have done fun things with. One below this is Drome Racers, the only Lego game I have that works flawlessly (= not Legoland) but have not completed and have absolutely no intention of doing so . I also have a copy of Soccer Mania but have never installed it.
(You can't ask a question like this and not expect a wall of text from me )