Giving advice is difficult. Macbeth is Macbeth - a very dark subject, but being an early opera is the most tuneful one. It requires a soprano of immense power but still able to sing in a refined way. The scene in which Lady Macbeth loses her mind in recalling her homicidal memories is a very powerful one. Probably this opera is the easiest.
Falstaff is Verdi's last opera; it is a brilliant piece but music develops on its own, tunes are very short and you won't be able to remind any one of them after five minutes. The Italian text is very brilliant but, alas, in Italian. Verdi was so old and independently minded that it could defy any opera convention.
Peter Grimes was written in 1945 and is one of the few operas written in 20th century that entered standard repertory. On a first glance it looks the most difficult one, but it should not necessarily be. The subject is terrible, a fisher who regularly loses at high sea his young boy apprentices, so it is progressively ostracized by his village till he gets mad and disappears, probably drowned at sea. If you can stand it, the opera develops much like a film, has some brilliant moments, it is in English and may be the easier one to understand as it is closer to our contemporary sensibility. The scene in which Peter goes mad is a sort of parody of great opera madness scenes - Peter holds the scene for seven minutes singing alone, accompanied only by a distant choir and a foghorn.
After that, you may have a look on youtube and decide yourself.