Well, that depends. If you are talking about SC models, Kawasaki has been an issue. BUT, if you are talking about NA models, they are more bullet proof than any other.
I would frankly place Yamaha a close second. and Seadoo in third. But it is somewhat engine specific.
Yamaha has two motors. An 1100 and the 1600. Nearly all rental companies use the Yamaha 1100 for three reasons: fuel economy, not too high performance, and reliable. The 1800 is just too quick as is the Kawasaki 1600 motor. For someone who wants a reliable, efficient ski and does not care too much about performance, you can;t beat a Yamaha 1100 boat. But if 50 mph is not fast enough for you, you can jump up to the 1800 or go with a Kawasaki 1600 and be in the low 60's all NA....no supercharger.
The issue I enjoy with the Kawasaki models is the Kawasaki STX 15F is likely the biggest bang for the buck, most fun ride you can find for the price. The Ultra LX has the same power plant, but sits on a larger hull, has more space for storage and is a bit slower due to weight and displacement.
Its sometimes hard to line-up model to model by the different manufacturers.
Seadoo went through a couple of years with some major issues with SC models which appears to have been fixed and Kawasaki's latest models which are now 330 hp I think, have also addressed some significant issues too.
I know I am rambling, but we have a local dealer here (very large) who sells all three manufacturers. Every sales rep rides a Kawasaki whom I have asked, yet their Seadoo sales and Yamaha sales dwarf the Kawasaki models.