I would say Eton or Harrow. Have you considered Radley if you're looking at all-boys full-boarding? I think it's a really lovely school.
Tonbridge - I've heard good and bad reports about the boys there, eg one shopkeeper told me they were always charming and polite, the mum of a boy who went there said he became an overprivileged d**k. I've been actively seeking opinions of all the schools, and its the only one where I've had negative responses. Another thing I heard from a mum was that there are a lot of wealthy children from London who like to show off their latest expensive electronics, boast about their possessions etc. If you are not one for giving your children these type of gifts on demand, they might feel very left out.
However, that said, the boy/young man who showed us around was perfectly nice. Talkative, informative, didn't seem overly privileged. He seemed to be enjoying it there and was sad to be leaving. Academically it speaks for itself, they had a very impressive number of all A/A* A-level results this year and the boys work really hard. Some of their facilities are fantastic, the science labs and D&T really made an impression.
I have heard the mutterings about Tonbridge and bullying. I don't know the specifics but you can glean some information by searching on Mumsnet, and the comments seem real. When I tried to address the concern directly at an open day, I was shut down and asked to speak to them in private. Their first question was 'Has your son been bullied?'. No, but that's not the point of the question...I don't want him to be bullied, clearly! I was given a spiel about their bullying policies but I wasn't very taken by their response compared to the very specific responses from other schools when I asked the same question. Radley for example, responded very well ( it's known for its excellent pastoral care).
Another thing I found unusual about Tonbridge was on visiting one of the boarding houses. I have visited 6 boarding schools so far, and it was the only boarding house where the bedrooms were an absolute mess! Underwear flung across the room, drawers open and clothes half hanging out, dirty mugs etc. Several of the parents on the tour commented on it. I imagine this is down to the ethos of the particular house master and matron, and we did only visit one house, but considering the bedrooms at the other schools have been either only a little messy or very tidy it did stick in my mind. At the other schools there were sometimes unmade beds, shoes strewn about etc but on the whole pretty good. I know it has nothing to do with the teaching, but I still wonder why those rooms were allowed to be such a tip.