Malefic and Benefic Planets

Planets and houses both have been categorized as benefic or malefic based on their disposition. The malefic or benefic nature of planets can be further classified as "natural" based on intrinsic temperament, or "functional" based on house lordship, and conjunction in case or Mercury.

Houses signify different facets of ones life and are benefic except for 3rd (mildly malefic), 6th, 8th and 12th house. Lordship of 6th, 8th and 12th houses make planets functional malefic. 

Houses have been discussed in details here.

Benefic implies the softness, generosity, prosperity, kindness, positive influence and a favorable period during the dasha of the planet.  Natural Benefics - Jupiter, Venus, Mercury and Waxing Moon

Malefic implies the harsh, cruel and punitive side where lead to diseases, prolonged illnesses, accidents, sorrows, worries, depressions and an unfavorable period during the dasha of the planet. Natural Malefic - Saturn, Mars, Waning Moon, Rahu, Ketu

Sun though considered harsh and cruel also signifies benevolence and warmth, hence shows both tendencies. In Scriptures, its considered Strong Malefic, but I leave it to your judgement.

Funtional benefic - Planet is considered to be a functional benefic when it lords the trine but is simultaneously not lording 6th, 8th or 12th house. Only in case of Gemini and Aquarius the lordship of 12th house is overlooked.

Functional Benefics for the Lagna - 

Aries- Mars, Sun and Jupiter
Taurus- Mercury and Saturn
Gemini- Mercury and Venus
Cancer- Moon and Mars
Leo- Sun and Mars
Virgo- Mercury and Venus
Libra- Saturn
Scorpio- Jupiter, Moon and Sun
Sagittarius- Jupiter, Mars and Sun
Capricorn- Saturn and Venus
Aquarius- Saturn and Venus
Pisces- Jupiter, Moon and Mars

Functional Malefic - Planet is considered to be a functional malefic when it lords any of the Dusthanas - 6th, 8th or 12th house. Also, both of planets sign in angular houses creates Kendrapati Dosha (KP) for the planet and the planet performs as functional malefic for the native. KP happens when Lagna falls in any of the dual Signs, but the planet also lording Lagna is exempted from the dosha.

For example - For Gemini, both Mercury and Jupiter lord two angular houses each; since Mercury also lords Lagna, KP is only considered for Jupiter, who becomes functional malefic for the chart.

One important aspect to consider here is when the Mool-Trikona sign of the planet falls in Dusthana, the severity of its malefic nature increases; when its other sign falls in Dusthana, its mildly malefic.

For Example - Venus is functionally more malefic for Pisces Lagna due to libra sign falling in 8th house, as compared to Libra lagna where though taurus falls in 8th house, but the mool-trikona for Venus falls in lagna. Some even consider that planet is functional benefic as far it lords Lagna, but my books suggest that lordship of any Dusthana has prominent impact on the native.

Scriptures suggest to NOT tag luminaries as functional malefic but with my experience Moon is highly functional malefic for Sagittarius Lagna, hence I include luminaries as well.

Functional Malefics for the Lagna -

Aries- Mercury, Mars and Jupiter (Mild)
Taurus- Mars, Venus and Jupiter
Gemini- Jupiter (KP), Saturn (Mild) and Venus (Mild)
Cancer- Mercury, Saturn and Jupiter
Leo- Moon, Saturn (Mild) and Jupiter (Mild)
Virgo- Jupiter (KP), Sun, Mars and Saturn
Libra- Mercury, Jupiter (Mild) and Venus (Mild)
Scorpio- Venus, Mars and Mercury (Mild)
Sagittarius- Moon, Venus (Mild) and Mars (Mild)
Capricorn- Sun, Jupiter and Mercury (Mild)
Aquarius- Moon, Mercury and Saturn (Mild)
Pisces- Saturn, Sun, Mercury(KP) and Venus