What is the easiest army to paint in Warhammer Fantasy?

This all depends on whether "ease" is through volume (number of miniatures) amount of colour or detail. Each criteria is broken down below:

Volume: Armies with high points value per model mean less miniatures to paint. Collecting Choas Warriors, Ogre Kingdoms and Brettonians generally means you'll have less minis to paint.

Amount of colour: Some armies can appear more presentable with less colours than others. To bring an army to a table that looks finished, you should use at least four colours to cover areas of skin, metals, wood/leather/hair and uniform. Greenskin armies can be brought to minimum table standard quickly after a black undercoat by applying green for skin, a single metallic colour for metals, a single brown for all leather, wood and hair and leave clothes and boots as undercoat black (touched up if required). Fully armoured armies like Chaos warriors can be done with even less! Skeleton units are also one of the easiest. Small miniatures like goblins and dwarves also need less paint. Ogres and giants need plenty of paint and need shading and highlights to prevent them from looking flat.

Detail: If you don't like the minute detail, avoid those models that need the extras to look complete. Bretonnian often don't look right without the little additions. Dwarf beards, slayer hair and beastmen can take multiple layers to get that hair/fur just right. Chaos Daemons and Lizardmen need some good blending to bring out their best.

RECOMMENDATIONS FOR NEWCOMERS:

If you get bored with painting big blocks, go for low number armies: Hordes of Chaos, Ogre Kingdoms.

If you are happy to pump out whole units in one evening by using production line methods: Most armies except Brettonians

If you get bored with painting small details: AVOID Bretonnians, Dwarves and Tomb Kings.

Best to avoid for newcomer painters: Bretonnians (difficult to pump out whole units of knights due to their individual paint styles); Wood Elves (the limited pallet of greens and brown can be difficult to get looking good); Lizardmen (the larger areas of skin can look bland without some blending, shading and highlighting.

A 100% Goblin army looks fine with minimal colours and are some of the smallest minis to paint and is recommended for building a big army fast. Hordes Of Chaos and Greenskin armies are close runners up.

This guide is for painting only and needs to be balanced with the ease of playing each army to select a suitable army.