The reason why there's different answers and opinions regarding this question is that when looking at basicity, you have to consider solvent effects. Therefore, you'll find different answers depending on the solvent used. In the gaseous phase (where there is no solvent) which is the truest measurement of inherent acidity/basicity, basicities follow 3>2>1>NH3. In some solvents the trend might be 2>3>1>NH3. The MCAT is not interested in testing you on PhD level theory. In all liklihood, you wouldnt get a question asking you to rank trimethyl, dimethyl, methylamine, ammonia. You should however, understand that alkyl groups are e- donating andwill therefore increase the basicity. Therefore, dimethylamine is always more basic than ammonia. And both of these guys are more basic than aniline. These are the types of important concepts to know. You guys seem like you already understand this quite well. Good luck with the mcat. Keep it simple and you'll rock the ochem questions.