Tough call. I'd probably say Middlebrooks, but you've got 3 back-up CI's that are roughly of similar value (Middlebrooks, Hosmer, Dunn) and Middlebrooks is your only other 3B.
All 3 of those guys are going to go hot and cold all season and are currently ice cold.
If it's a keeper league, I'd drop Dunn. If it's not, I'd probably go ahead and drop Middlebrooks (unless the waiver 3B options are atrocious, which is possible).
10 team 5x5 ROTO (OBP), 11-player keeper, C/1B/2B/3B/SS/MI/CI/IF/5OF/UTIL/8SP/3RP
C: Castro, Montero
1B: Rizzo, Adams, Lind
2B: Kinsler, Kipnis
3B: M.Cabrera, Frazier, Reynolds
OF: Stanton, Trumbo, Choo, Marte, Rasmus, Blackmon, Bradley Jr.
SP: Gio, Scherzer, Ryu, Cashner, Kuroda, Lackey, Minor, Wood, Peralta
RP: Balfour, Perkins, Nathan, Frieri, Fields