A more formal equivalent to smarmy would be unctuous:
: revealing or marked by a smug, ingratiating, and false earnestness or spirituality [m-w.com]
And an alternative for slick would be suave:
: smoothly though often superficially gracious and sophisticated [m-w.com]
: winning over by wiles [adapted from m-w.com]
beguiling [to me, this one seems closest to persuasive]:
: engaging in the interest of by or as if by guile • His seductive voice beguiled the audience.
: leading by deception • beguiled into ambush
: deceiving by wiles • had intended to beguile
[all adapted from m-w.com]
: influencing or enticing by soft words or flattery
: gaining or getting by wheedling • wheedle one's way into favor
[adapted from m-w.com]
To me, all of these carry at least a whiff of negative connotation.