In order to accurately answer that question I'd need to have owned a car or two from every manufacturer. I've driven several cars, but... I can't base my opinion of the whole company on that.
That said... both my best friend and I have been very happy with our Mitsubishis (04 ralliart, 03 ES), but .. we've only had them for a short period of time, so I can't say anything for long term reliability just yet. I haven't had any problems to this date however, and it's been about 1 year since my lease started.
I also had minimal problems with my Camaro that was 20 years old, before it got stolen. I hear lots of people bash both GM and Mitsubishi... but having driven/owned both, I can't say I despise either. However, I can definitely say the service at Mitsubishi is among the worst I've ever seen, and this is based on SEVERAL dealerships, not just the one I bought my car from. Dealers that I found treated me better were one Honda dealer (not all), and JEEP.. they were very good.
Kia's service was horrible when I went, and I've heard some horror stories about those... but again I can't really judge.
My next car definitely will not be a Mitsubishi, Hyundai, Honda, Mazda or Toyota. Simply not satisfied with any of their service to the point of making me want to purchase their car. Product isn't good enough for the price either, IMO... Although I still haven't seen a deal better than what I got with Mitsubishi. I think the base Corolla and the Mazda3 are the only cars that come close to offering the same value.
Worst automaker overall? Can't say..