What is the best army in warhammer 40k?

Answer
For Warhammer 40K but there isn't really a best , they all have pros and cons. Usually Space Marines are very strong or chaos for fantasy but it all depends on what you like go with your first instinct.

Here is some good tips: Firstly choose an army that you think looks nice or you like the looks of. Then pick up the codex of the army and have a flick through it. If you start to like the army, vehicles and the characters, then start planning your army. Cut out some circles and write down some of the characters on them etc. Eldar harlequin with 5 guardians. Don't just go straight into buying because you might not actually want or need that item that you bought. After planning start buying your characters that you think are good in a battle of Warhammer 40k and start owning with your army.

AnswerThen again it is also the type of tactics you will use in a game of Warhammer 40k. For example, if you like to be the stealthy type team, Eldar are particularly good. Research your team before buying any thing. Most Warhammer shops allow you to have a read through their books. Each army has it's upsides and down sides