The hot water and bubbles...oh I also love the location of my spa (my back yard).
I've always enjoyed soaking in hot tubs at hotels, apartments, friends houses, etc. Make & model really didn't matter all too much (we always had fun and enjoyed ourselves).
Having said that, I was a bit more selective when it came to buying my own hot tub.
If you're looking for another answer...here goes:
I like the upper back/neck jets. A very close second is the foot jet.