On our 2003 off property trip, we only ate at one table service restaraunt out of the 4 days we went to WDW parks.
In 2004 on another off property trip, we only ate at 3 TS places, and in 2005 we only went to Epcot and ate at one TS restaraunt.
In 2007, we stayed on property and made sure to eat all our TS meals at restaraunts we hadn't eaten at yet.
So, thus far, Mama Melrose is the only TS restaraunt we have eaten at more than once (2003 and 2004), and it is also the only restaraunt on our 2008 lineup that we have already eaten at. Not because it's a "must", but out of the TS places at DHS, that's just the one that my wife wanted to eat at next time.
As far as counter service goes, I would probably consider Pecos Bills a must.