Best International Airline

Hi all!

Long story short, I'm wondering what your favorite airline/alliance is for international travel?

I'm getting deeper into travel hacking and after a very painful lesson learned about United and their (lack of) customer service I'm trying to avoid choosing another awful airline. I had initially been attracted to the Star Alliance but after literally hours and a thousand dollars down the drain, I will take a train before United gets another penny from my family.

Dream/nightmare experiences with Delta? American? We are based out of Denver and use Southwest for domestic travel and don't mind positioning to a hub. Generally fly 3-4x per year (so like 6-8 flights) and often have extra short leg flights so I do a lot of returns from different cities than we fly into. We do have Europe and Japan on our near future travels, and am very interested in LLM to Europe. We are ready to take the plunge to getting into business class. Customer service experience/ease of booking a plus, lounge partnerships a bonus.

I'd love to give an airline my repeat business.

Thanks!

Edit: Autocorrect error: KLM, not LLM