Looking at others in the show ring, I find that the nicest looking presentation follow the "KISS" philosophy: Keep It Simple, Sweetie.
To me, neither Western or English really enter into a halter class, at least not when done to excess. However, the two nicest turnouts I've ever seen were: 1) Black top and pants with a tailored royal blue jacket, black gloves and black Western hat. This was at a rodeo, and this woman outclassed all the bling around her.
2) A young woman wearing a long, slim black jacket with a slight English look to it. Very flattering, very slimming.
If you go with black, be sure to choose a fabric that does not attract dirt, or is easy to dust off before entering the ring.
Look for something that resists wrinkles, or you'll look like you slept in your clothes by the end of the day.
I love pants that have just a bit of stretch to them...and also those that are slim fitting but not tight. Have them hemmed so they don't bunch up over the foot.
Don't do as I have done -- I have too many photos where my horse looks great, but I look frumpy. Of course, having a tall, lean horse tends to make me look short and dumpy no matter how I'm dressed, LOL!
Another area where I'm an example of what NOT to do: I'm always so wrapped up in how my horse looks that I've actually forgotten to brush my own hair! Tie your hair back or choose a hairstyle that will look good all day, won't hide your number and won't get in your eyes.
Stand up straight...look at photos of Erica, Annete Helgerson, Lori Guglielmo, and others who always dress in a simple yet classy manner and present themselves AND their horses beautifully.
Complement your horse -- don't distract from them.