kimberley: (to arnold) welcome little brother
arnold: whatcha smokin?

drummond: i've never seen a white goldfish before
arnold: that's ok. he's never seen a rich white man before either :lol:

arnold: if this was heaven, i'd kill myself tommorow

and the ultimate...
arnold: whatcha talkin' 'bout willis? :lol:

Maggie (All of these happen to be Dixie Carter's Maggie)
My Stars and Gartars in 3 episodes-Tonsils when Mr. D messes up his knee and has to have surgery, A Camping We Will Go when entering the apartment with Arnold and Sam (looks like she picked them up from school) to find the living room full of camping equipment, and in Carmella Meets The Gooch when Arnold returns home from talking with the Gooch sporting a baseball bat around his neck.
I'll give them a backside full of buckshot-Russian Embassy
In the episode Tonsils Sam is hiding under the sheet because he is freaked out over having to have his tonsils out and Mr. D is talking to him facing the end of the bed that he normally puts his head on and Maggie says You're talking to the wrong end, the part that listens is down there and points to the other end of the bed where his feet normally are.
In the episode A Haunting We Will Go-Arnold you scared the britches off your little brother. His imagination won't catch up with him until next week and Hon, lets go tuck Pearl in (freaked out about the boy's scary story Pearl leaves the living room and announces that she is going to sleep under her bed; after lecturing the boys, Maggie turns to Mr. D and says this line)

actually it was a black goldfish

Philip: "Well, what's in a name?"
Arnold: "I don't know what's in a name, but I know what's in a crock."

Mrs. Garrett: "God helps those who help themselves! And everybody else is up a creek..."

Mrs. Garrett: "Always do your best, Nancy, no matter what!"

Philip: (meeting the boys for the first time) Welcome, Gentlemen!
Willis: (confused, looking behind them) You talking to US?
Philip: Of course.
Arnold: How 'bout that, Willis? Downtown 2 minutes and already we're gentlemen.