I'm not even considering the current tour, nor the Dance one. If they aren't supposed to be a nostalgia act then why do they act like one?
I guess the 1975/1976 tours were ideal, for the reasons ChiliD mentioned. Lindsey may still think they sounded like a lounge act on other people's tunes, but I guess it's mainly because they didn't do enough of his songs.
But otherwise the line-up didn't really hit its stride live until the Tusk and Mirage tours. The setlist may not have changed much, but they could still pull up a surprise from the sleeve ("That's Enough For Me" once in Europe, right?) and the arrangements and solos varied enough. It wasn't the most pleasant time for the band members, in fact, maybe that's why those two tours had the youthful intensity at its max.
The Tango tour I have problems with: the backing musicians and singers appear for the first time. The band interplay begins to lessen. Stevie's voice is at its worst; I even prefer her tone on "When I See You Again" to these flat performances that she gave. The change in her stage movements was disappointing too.
But on the other hand Rick and Billy sounded more comfortable with the PG era material than Lindsey (with the exception of "Oh Well"). So at that point one could still maintain illusion of FM retaining the band interplay of the past.
Tusk it is.