I personally think anesthesia or radiology would be the most helpful to you; getting some experience intubating and placing lines and IVs (an early in the year SICU rotation may not get you a lot of procedures if new interns/residents need them, plus you get to spend time in the OR rather than the wards), or looking at films to boost your confidence as an intern at identifying more basic radiologic findings. My school's radiology rotation was both helpful and short hours-wise, so it was very popular; anesthesia was popular with surgeon-types because they let students do stuff. Ask around and see how your school's rotations are.
While GI or cardiology are good rotations depending on your interests, the hours of IM subspecialty clinic (IBS patients/hemorrhoids/CHF patients/Cardiac stent patients) or rounding (if on the consult team) may be...painful and less helpful to you clinically, especially later in the year when you are just waiting for interviews/the match.