#2 I recommend memorizing these words. Suggestion: I recommend that you memorize these words. If you use #2, to make it sound more natural you need to follow it up with something. For example: I recommend memorizing these words so that you will have a larger vocabulary. Hope that helps :)

"1) I recommend you memorizing these words" --> "I recommend that you memorize these words." ----> The gerund/present participle (not neccesarily the same as "gerundio") is not appropriate for something you are directing someone else to do, you are directing them to do something now or in the future so it is not active "2) I recommend memorizing these words" This is okay because the subject is not clear, this works in spoken, but I would avoid in writing. "3) I recommend to you memorizing these words" Similar to "1)". This would not sound too awkward in speaking, but in writing it would not look strange. The infinitive might make more sense here (as Ted suggested in his answer) because you are recommending the action itself.