While it was intended by the developers for people to "decide from themselves, there is no right or wrong answer", the interest in Tifa is blatantly in everyone's faces while the "interest" in Aeris is left as merely a possibility. Really, the most obvious showing of love between Aeris and Cloud most certainly comes from Aeris, and she is merely confused as she sees a lot of Zach in Cloud due to Cloud taking on parts of Zach's life/personality during the ending of Crisis Core. Once Cloud knows who he really is, Aeris is already dead (and, as we see in Advent Children, happy in her death as she's now with Zach). On the other hand, Cloud finds himself having sex with Tifa. Some people might go around suggesting that they didn't actually have sex, but I really think those people need to break out of their denial - they obviously did, but it's not like they were going to plaster the words "THEY ARE HAVING SEX" all over the screen.

Alternative answers of Barret and Sephiroth are most certainly acceptable.