toyota, only because of lexus, and even then i dont like the cars much, i rather go german.
mazda, well ive had them catch fire on numberous accounts, although those were older cars, some with low miles, some with high... electrical was a problem, and getting the doors to open, they all seemed to mess up with the doors. sense most were older modles i would give mazda another chance in a with the 3 and 6.
nissan, well ive owned some newer models, ive spent well over 2000$ on a 3000$ car. im getting rid of it soon. its in perfact shape, but twice the engine stopped working (took it in, never figured out what was wrong, but got it working again) then it eats stearing fluid, and used to leak oil. i dont like nissans one bit. other modles have just been horrible reliabilty (familys cars) ive owned two, a truck 95, and a sentra 97, both under 100k miles, both had a lot of problems... never will i own another nissan, well not this decade...
If you are talking about the cars, I'd say Mazda (that's how I voted). As far as a company to invest in, Toyota is a monster. They are better off than any auto company in the world. Honda stakes out middle ground on both counts - cars are more interesting than Toyotas, more reliable than Mazda (overall), and their business is in good shape.
I love Nissan/Infiniti mostly because of Infiniti. But, I chose Toyota/Lexus/Scion as the Japanese Invasion because Toyota and Lexus are the first names that come to Americans when you say "Japanese car". Also, Scion has targeted their demographic perfectly IMHO.