/r/fatestaynight's Official Viewing Order Guide.

The unanimous Anime-only order:

1. Fate/stay night: Unlimited Blade Works[2014 TV]

2. Fate/stay night: Heavens Feel [2015-6 Film(s)] [Not yet released] (this may be skipped if it's a long ways off for you, but be warned Zero does spoil some aspects of HF)

3. Fate/Zero

4. Other (includes OVAs, Carnival Phantasm, and Studio Deen's 2006 verison of Fate/stay night [even though it's terrible, it still at least has portions of the Fate route you missed])

The Intended order:

1. Fate/stay night Visual Novel [roughly 60 hours long]

[After this point the order becomes very flexible, you can now follow the Anime-only order or this one]

2. Fate/Zero

3. Fate/hollow ataraxia Visual Novel [>50 hours]

4. Fate/stay night: Unlimited Blade Works[2014 TV]

5. Fate/stay night: Heavens Feel [2015 Film]

6. Other

If you want a more detailed discussion on the order, please view this thread.