Can I skip UBW and watch Heaven's Feel?
Can I just watch Fate/Zero instead or when should I watch it?
What is Fate/Stay Night and what order should I watch Fate in? Why does it have to be so complicated?

Simple Breakdown:

Fate/Stay Night is one story that is told in 3 parts--

0. Fate/Zero (Ufotable)
1. Fate/Stay Night-- Fate route (2006 by Studio Deen)
2. Fate/Stay Night-- Unlimited Blade Works (Ufotable)
3. Fate/Stay Night-- Heaven's Feel (Ufotable)

Each story has its own point in explaining the plot.

1. Fate route-- Realizing that your ideals are-

- flawed.

2. Unlimited Blade Works-- Confronting the problems with your ideals face to face.

3. Heaven's Feel-- Discarding the flawed ideals that Fate and UBW presented to pursue something more valuable.

All 3 of these are in a sense Alternate Realities that connect to tell -

-a single story and teach one big lesson.

So where does Fate/Zero fit?

While Fate, UBW, and HF are told in the same time period, Zero is a prequel to this story.

0. Zero-- It presents the concept of ideals and failure. It is the "what if?" situation to what happens if someone-

-continues to live a life of self-destructive ideals and self-hatred. It's a situation in which nobody wins. Sacrificing all that you have, to realize that your "ideal goal" was false. It brings up the idea of your ideals failing you

So where does this all fit into the plot and-

-where should you watch it?

In my opinion, it is OK to watch Fate/Zero anywhere before Heaven's Feel. Heaven's Feel answers many of the questions presented in Fate/Zero and plays off of much of your knowledge from it.
You CAN watch it after Unlimited Blade Works, but it's bette-

-r to watch it beforehand because Ufotable adapted UBW with the idea in mind that you had already seen Zero. So it references Zero as well more than the Visual Novel and even adds an additional scene to the epilogue relating to Zero.

A watch order?:

0. Zero (before or after 1)

1. Fate (2006)-- people tend to complain about this adaptation, but overall it's a good adaptation that only falls short in animation as well has a few extra scenes that are just as easily skipped because they hold no bearing on the plot.
2. Unlimited Blade Works
3. Heaven's Feel

If that wasn't enough to get you interested.
Why should you watch Fate?

Cuz hot

Yes. I know. I know.

"Read the VN!"

I'd rather not gatekeep people who refuse to read one lol

"There's no perfect way to start Fate that isn't the VN!"

Nasu created many entrypoints. They all have their drawbacks, but why push away fans because they don't read the VN?