Usually when I look at a new class, I envision what class would be closest from the core.
Along those lines, if it uses alot of spell casting (psion, warlock, beguiler), I compare it to a wizard, so a psion is about the same power level, warlock seriously underpowered, beguiler's a bit of a weak enchanter with other tricks. So for the BAB guy its the fighter (though I'll sometimes use the barbarian), and for a guy who relies on useing skills its the rogue.
But if I absolutly had to choose ONE, it would probably be the rogue as the most balanced.