There are 3 oneworld lounges in Terminal 3 of London Heathrow, and they’re all pretty close to each other. My personal favorite is the Cathay Pacific lounge, which is reviewed here. (Note that this lounge is currently closed for renovations and is scheduled to reopen in June 2016)
The lounge isn’t huge, but I’m impressed by it. In the times that I’ve visited, it’s never been very crowded, so that may have contributed to my positive impressions. But there’s plenty of seating, and it just feels calm and serene.
The drink options are extensive for such a physically small lounge. The liquor selection might not be as good as the British Airways Galleries First lounge, but I don’t drink much, so that doesn’t matter to me. What does matter to me is that they have fun non-alcoholic drink options like coconut water, mango juice, and soy milk.
The food options are also extensive, and it reminds me of a mini Asian hotel buffet. For breakfast, they have English breakfast items (e.g. baked beans, eggs), a noodle bar, smoothies, congee, pastries, oatmeal, yogurt, etc. So lots of options, even though it’s a small space. The rice pudding I had was delicious, and the dan dan noodles were very peanut buttery, which I liked (even if it’s not authentic).
As for getting actual work done, there were a decent number of power outlets, and it wasn’t very crowded, so there’s lots of space to spread out. The wifi speed was decent with about 3-4 Mbps down and 1-2 up.
Again, this is my favorite oneworld lounge in LHR Terminal 3. Maybe it’s just because I’m Asian, but I think this lounge has the best food options, and it’s not as crowded as the other oneworld lounges, so it’s the most peaceful and pleasant to just sit in.