We have owned a good many System 11 machines over the past 5 or 6 years and I would say Millionaire & Bugs Bunny Birthday Bash are two of my least favorites. Originally I didn't really like Road Kings (mostly because of the art) but that is a decent playing machine overall. Jokerz is kind of questionable - not one of the better System 11's but maybe in the middle of the pack.
As far as top titles for me personally I think Pin-Bot is the best System 11 deal out there and High Speed is a close second. Not the best two system 11 machines but as far as bang for your buck those two machines can still be bought in nice condition for around a grand to $1200 ( a steal of a deal for a system 11).
Whirlwind & Earthshaker are two of the best system 11 machines of the ones I have played if you are judging by gameplay and overall appeal. I would have to also include Taxi and Banzai Run on a system 11 "better" or "best" machine list.
Diner, LW3 and F-14 fall into the middle of the pack for me but lots of people seem to like Diner. We have also owner a couple Grand Lizards and they are pretty decent.
For the most part - dollar for dollar I don't think you can do much better than most system 11 machines. Lots of great titles under $2500 and a good many still under $1200 to $1500. System 11 machines are one of the best values in pinball today as far as I am concerned.