In my opinion, you only need to do 1-2 months of anesthesia, unless you still need good letters, then do another month. If your letters are all lined up, take something interesting.
That said, here is my view on some of the common electives-
ER - take 1-2 months of this if you're at a place that will let you do alot of procedures like lines, suturing, etc. This is also a great place to learn how to properly place IV's. Important since you probably won't be doing too much of that as an intern. This is one skill that even most CA-1's and 2's struggle with.
Cards - bread and butter of anesthesia, especially arrhythmias. 1 month will be good. We end up being the cardiologists in the OR.
ICU - honestly, this is probably not that high yield unless you are at a place that makes you take overnight call with the residents. This is important as overnight, it's often just you and 1-2 others, so you actually get your hands dirty, as opposed to watching during the day when there is more that enough help around. Helps a bit for internship, but another month of IM would probably be more high yield.
Pulm - kind of low yield. Most pulm rotations focus on CF, sarcoid, etc. You should be able to learn about this at a decent IM internship. That said, if you plan to do a TY, this might not be too bad. You will probably never see a vent on this rotation. Maybe a bronch, but any competent CA-2 can bronch a patient.
Heme/Onc- actually a pretty high yield rotation. I mean, you'll be seeing a lot of bleeding in the OR with a lot of dyscrasias. Not to mention the fact that a vast number of your OR cases will be Onc related. Plus, the schedule is usually nice.
Radiology - will help for internship, not so much as an anes resident. Yes in the ICU, but you'll get enough as an intern for that.
Elective rotations in surgery - if you like 80 hour weeks, knock yourself out. I mean, yeah, trauma surg sounds like fun, but if you're the ER medical student, you see the same thing, without having to go to the OR where you'll spend 4 hours holding retractors.
That said, ortho rotations tend to be pretty good if you can put up with the personalities. They usually let you do a lot of stuff in the OR and will actually value your input (since you as a med student probably know more about internal medicine than they do - after a few ortho preops, you'll understand).
GI/ID - usually nice schedules if you have an interest in the topics.
Endo - OK, there are really only 3 things you need here - Adrenal insufficieny, Pheo's, and Diabetes. Yes, there is more to it, but it's so low yield for anesthesia. Don't spend a whole month on FNA and thyroid stuff that you will never use.
Wilderness med - just a fun rotation - wish i did it in med school.
Just my 2 cents...