I think Bally's score motor design is actually quite good. It is highly serviceable, and everything is generally accessible and easy to find.
The Gottlieb games, to echo what most others have said, are the best made. Great feeling games.
In the bingo world, there were two main manufacturers, Bally and United. The United games are a similar jump in quality 'feel' over the Bally games. I was very surprised to feel this difference when I picked up my (only, at the moment) United.
Any Gottlieb short-throw relay with the short switches is very difficult to adjust properly, and anyone that jumps into these games typically creates problems by adjusting a single switch and not verifying the position of every other switch on the relay. You don't really run into this with Bally or Williams. But, the short throw, when adjusted stays in adjustment well, doesn't attract as much dirt, and doesn't arc nearly as much when dirty (typically). 'Course, part of that's the 24V instead of 50V circuitry.
I really don't like a lot of the Williams units, but their score reels are pretty easy to work on. Bally's, more so.
Williams' score motor is a bit frustrating, though (to me). Lots of metal and very easy to accidentally snag a switch.
I think the Bally units are the best of them all. Their stepper design is very simple and serviceable. The nylon gears are easy to clean and the stepper itself is easy to adjust.
The Gottlieb score motors, as someone else said, have bad switches on the inside, but that's because everyone fixes those external switches and lets the rest rot. Once a Gottlieb unit is tuned, again echoing most others, it stays in adjustment for a very long time.
I've only worked on two Chicago Coin games, and each was a little quirky, but mostly Williams parts inside, so I can't really comment.
Genco and Exhibit games are also great! I like Exhibit a bit better (from a service perspective).
I really dislike working on a early-mid 70s Williams. The way they installed their relay banks under the playfield is often the perfect place for wires to get snagged and so forth. Not that I can talk much about that with Bally's shutter motor switches...