Sterically unhindered bases are generally good nucleophiles. Nucleophilicity is something that you learn to evaluate through experience, after having seen a bunch of reactions. There are a few rules that generally work, like:
1. More electronegative atoms are less nucleophilic, as you said. Thus, alkoxides are more nucleophilic than flouride.
2. In most cases, in polar protic solvents, ions with larger radii are better nucleophiles, because smaller ions are more strongly solvated. Thus, thiolates are better nucleophiles than alkoxides. In polar aprotic solvents, ions with smaller radii are sometimes better nucleophiles.
but these rules are general extrapolations from observations on many different nucleophiles.
If the MCAT were to test you on your nucleophile skills, I have a hunch that they would isolate one quality of good nucleophiles. For example:
Which is the best nucleophile?
A. CH3CH2O-
B. (CH3)2CHO-
C. (CH3)3CO-
D. CH3CH2OH
On the other hand, it's conceivable that the MCAT asks you to identify the major product of a reaction where two nucleophiles directly compete. For example:
When CH3CH2Br is in the presence of equal amounts of F- and CH3O-, what is the major product?
A. no reaction
B. CH3CH2F
C. CH3CH2OCH3
D. CH2=CH2
In this case, you'd evaluate, among other things, which nucleophile was the better leaving group.
Answers: A and C
