The EPA's fuel-economy categories bear no relation to their standard size categories, or apparently any relation to reality at all. Beside no-one said most fuel efficient vehicle, but more vehicles in more categories.
GM lags in compact pickups true, but in fullsize pickups is the leader by far. Sierra and Silverado V8 hybrids compare with 6-cylinder compacts, and the standard 6-cyl Sierra and Silverado aren't far behind, ahead of the lighter, smaller Toyota Tundra. 4wd fullsize pickups, again it's the GM hybrid pair. Standard V8 fullsize pickups, again it's GM with the Silverado and Sierra, in both 2wd and 4wd. The only GM fullsize pickups not to beat the V8 Tundra are the 6.0 L Classics.
SUVs and crossovers? The Saturn Vue hybrid matches the Highlander and Rx400 overall, behind the Mariner and Escape. Among compact crossovers the l;ead is taken by the Patriot and Compass. In the D-segment, the RAV4 takes the 4-cyl and 6-cyl, 2wd & 4wd lead. GM has no entry against the 4-cyl Highlander, but the Equinox, Torrent and Rendezvous all beat the Highlander V6. Toyota wins midsize SUVs with the 4Runner, GM owns fullsize SUVs with the V8 Tahoe and Yukon beating the V6 Trailblazer. Even the Suburban and Yukon XL beat the Sequoia, and the only SUVs with worse economy than the LX470 and LandCruiser are the AMG ML63 and the GRand Cherokee SRT-8 which at least have the excuse of producing substantially more than 400 hp, instead of substantially less than 300.
Toyota beats GM in fwd V6 minivans, but is no better for 4wd models.
The Solara just beats the G6, in both coupe and cabrio.
The Scion tC is handily beaten by the Ion, Cobalt and G5, which shows that GM really needs a smaller engine to beat the Corolla.
Avalon beats the Impala, by 1 mpg in city driving.
For midsize 4-cyl sedans, Malibu and Accord autos match the Camry manual and beat the Camry auto. The best V6 is the Malibu, just ahead of the Camry.
In luxury cars, the ES350 beats the Saab 95, while the GS350 beats the CTS and STS V6s, but the GS430 gains 1 mpg in the city and loses 2 mpg on the hwy compared to the STS V8.