Before addressing System 11's, you might also want to consider World Cup Soccer (WCS). It's a lower priced DMD with a rule set a little deeper, but similar to FH and the System 11's. It's a great family game with the sports theme, cheering crowds, etc. and basic rules that are easier to explain than other DMD's from that time frame or since then. Has two great multiball modes (Final Draw - you try to win your way up in rank from current seeding position by beating teams to the point where you get to play the #1 ranked team, and World Cup - traveling through the cities to make it to LA to play in the World Cup against Germany)

As to System 11's, I owned several and have played all of them a lot. The ones I would consider adding, in order of personal preference: WW, Taxi, Pinbot, Space Stn. And, to reinforce what others have said, WW is the BEST system 11 unless you don't like games that are very challenging with multiball that is very difficult to start and with hard to score jackpots during multiball.

WW and ES are similar in rule set, but I'd place WW over ES because ES has rules that are flawed and WW also has cooler (literally - get it?) toys. Taxi and Diner are also similar games, and, although Diner is prettier to look at and has slightly deeper rule set, Taxi is just more fun to play (and usually less $). Space Stn is similar to Space Shuttle with some added rules and a very cool (especially with LED lights) special light show for multiball - Condition Green. There are several other fun System 11 games, and I can see why they might also be preferred over the ones on my list by others, but just providing my preferences here.