If you're talking about from the Executive Selection (ES) at aisle locations, I would never EXPECT to see higher than an Impala 2LT/LTZ or Maxima (both PCARs) in terms of sedans on the ES... I've seen Lacrosse or 300 on ES before, but rarely. The best upgrades you're likely to find on ES, in terms of interior luxury, are the the newer Tahoe/Yukon, Grand Cherokee Limited, and Durango Limited... I see these on ES far more frequently than the better PCARs.
That said, as an EE, why not just reserve PCAR? The price difference from ICAR to PCAR will be marginal as you'll be paying the FCAR rate. You still could be stuck with a Maxima or Impala 2LT, but you'll give yourself more leeway to ask for a better PCAR (even if it means bringing something up from storage) and also be more likely to get an upgrade to a LCAR/GXAR, ESPCIALLY when the station isn't correctly operating ER-PREM/LUX which gives you even a bit more leverage in negotiating a better car.
Remember, all an ICAR res with ES guarantees you (per the T&Cs) is an FCAR or better. It's often possible to do better, but if you expect or need better, reserve the class you need/expect.