at its best , which occurs often 
(ROOT (PP (PP (IN at) (NP (PRP$ its) (JJS best))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBZ occurs) (ADVP (RB often)))))))
(ROOT (PP (PP (IN ) (NP (PRP$ ) (JJS ))) (, ) (SBAR (WHNP (WDT )) (S (VP (VBZ ) (ADVP (RB )))))))