It's always interesting, as everybody has their own views on such things. (Although certain things, such as UA's inferiority, appear to be universal.) I, for one, disagree with those who seem to really like TG's BKK lounges. I am not much a fan, myself. I think that BR (post-renovation) is the best lounge in BKK by far. TG tends to have a very poor food and beverage selection.
For those interested in LHR rankings, this is my two pennies worth.
If you don't need to go to 2B, stick with Lufthansa. They have a nice lounge. Good food. Nice atmosphere. (However, their internet makes your computer/device think it is in Germany, so if you are using anything which requires you to be in UK or location based system, this could cause issues, so plan ahead.)
If you are going to 2B, you have 3 options
Air Canada-Don't. It is tiny. Very few seats. Decent view, but you can't tell, because you can't sit down to see it. (Okay, so I am sure plenty will disagree, but when you have access to so many lounges, I see no area where this lounge is competitive.) They do serve a signature drink, which, if memory serves includes Canadian whiskey and Maple syrup. (Not my taste) Food offering isn't so bad. But despite large massive window, the colour choices and the layout make it feel small and dark. In all, it just didn't work.
United- Go here if you are most interested in a drink or a shower. They will press your clothes for you while you shower and also have very nifty little foldable toothbrushes. The bar menu is large. Food is satisfactory, but I don't remember it being phenomenal.
Singapore- If you are interested in some Singaporean cuisine, I would recommend the SQ lounge. They have my favourite food out of all of the options. Also some good drink options, including the ever-popular Singapore Sling.
As LHR is my home airport, when I fly from LHR, when I go shorthaul, I stick with LH. When Lounghaul, I will usually go to SQ, (Since it is now open) and if time permits or if wanting shower etc. I may go over to UA. Depends on timing. But for me, pretty much boils down to LH for Euro flights, SQ and/or UA for Longhaul. Anybody else with different thoughts?