Spoiler Alert: the movie-related Top Fives featured here obviously assume that you've seen the films in question or don't mind knowing about their most prominent moments so don't come whining to me if they ruin a film that you haven't seen yet!
5 - Gisele (Gal Gadot)
Introduced in the fourth film as an ex-Mossad agent and then the antagonist's bit of stuff, Gisele soon develops an interest in Dominic. He isn't interested of course, having only just discovered the death of his girlfriend, but it doesn't stop her warning him of imminent danger at the film's climax; a gesture repaid when Dominic saves her life. From thereon in she becomes a member of Dom's team, before long hooking up with the ever-munchy Han, forming a mighty useful team in the process. Happily, she's also featured in all subsequent films too. Hooray!
4 - Suki (Devon Aoki)
Sadly, the 'unique' looking Suki appears only in the second film, and even then she doesn't get a great deal of screen-time, but she certainly leaves an impression anyway! Her bright pink Honda S2000 remains one of the most eye-catching cars in the franchise and her crew of girl-power racing pals can keep up with the boys without even breaking a sweat. Despite that, though, her brief appearance in a striking red bikini is the scene I remember most!
3 - Neela (Nathalie Kelley)
The third film seems to have been all-but-forgotten by the franchise powers-that-be (with the exception of Han, obviously) but it was my favourite film in the series for a long time. One reason for that is the delightful Neela, the Aussie 'schoolgirl' (she was actually more than 20 when the film was made) who's tied to wannabe Yakuza oaf, DK. It's not long before her charms attract others though, notably DK's rival and fellow 'student', Sean (who was played by Lucas Black, 24 at the time).
2 - Mia (Jordana Brewster)
Good old Mia, the rather meek (for F&F standards) kid sister of Dominic who is initially used by the then-FBI agent, Brian, in order to infiltrate the street-racing world dominated by Dom. He soon finds himself interested in her for other reasons though, and who can blame him? She's hot, she can drive, and she sends his competition (Vince) packing in a rather cold way, giving him a clear sign that she's... willing. She's probably the main cast member who has the least action-packed role but she sure looks makes the backgrounds of most scenes look nicer.
1 - Letty (Michelle Rodriguez)
Ah, the mesmerising Ms. Rodriguez... She's long been not only one of favourite things about the F&F series but my favourite celebrity crush of all! The character of Letty is typical of the roles she usually plays too - tough, hard-talking arse-kicker who takes no shit from anyone. Of the main cast, she's probably had the least screen-time, much to my chagrin - she appeared throughout the first film and the start of the fourth film before sadly being killed off, and until recently that was it... until she magically came back from the dead to make a triumphant return in the recently-released sixth film - hooray! Happily, her arse-kicking and low-cut tank tops returned with equal prevalence too. I'm sure looking forward to the seventh film now...
Honourable mentions: Monica Fuentes (played by Eva Mendes) from 2 Fast 2 Furious, Reiko (Keiko Kitagawa) from Tokyo Drift, Elena Neves (Elsa Pataky) from Fast Five, Riley Hicks (Gina Carano) from Fast & Furious 6...