My vote would go to 1989, such a wide variety amongst the 20 teams

McLAREN: The White and Marlboro Red, an instant classic
TYRRELL: Began with a sponsorless but elegant all blue before adding Camel Yellow to the airbox, sweet
WILLIAMS: My most favourite livery of all time, the White, Blue and ICI Yellow with the Canon rear wing, awesome
BRABHAM: Classic 80's Brabham colours of white and dark blue
ARROWS: Cream and Burgundy, an odd mix but it worked well
LOTUS: Their Camel Yellow phase, not that its a bad thing, it just seemed a brighter yellow than previously, made it stand out a lot though
LEYTON HOUSE MARCH: Turquiose and BP Green made it easy to pick out of a crowd, very elegant too
OSELLA: Similar to Arrows except instead of Burgundy, they had red and orange Fondmetal stripes
BENETTON: The famous Rainbow livery, some say it was ghastly but I thought it was brilliant, certainly better than that mix of Camel Yellow and Pea Green in the coming years, yuck
DALLARA: A Ferrari clone livery, not a bad thing though
MINARDI: Their classic original racing colours of Black and Yellow, cant go much wrong with that
LIGIER: Plain and simple, All in Gitanes Blue with black wings, classic Ligier colours
FERRARI: Back when Ferrari's were in proper Ferrari Red and not the bright number like we see now
LOLA/LARROUSSE: Another multi coloured variety of Black, Yellow, Green and Red, not as memorable as th Benetton colours though
COLONI: Obscure but I though elegant, Bright Yellow and Blue with Green trim, then replaced with a more professional looking White and Black with Blue and Yellow trim.
EUROBRUN: My least favourite of this season, a horrible all Orange affair
ZAKSPEED: Classic Zakspeed F1 colours of Red with the White stripe
ONYX: All Blue with the White stripe and bright Pink trim, simple but sweet
RIAL: Two-tone Blue with big Yellow RIAL letters, not as good as the Benetton Mild Seven offering in future years
AGS: Black and White, similar to Minardi in 2001