I am at Reading right now, second year student, second year living in halls. Currently writing this in halls.
Mackinder, Stenton, Childs, Windsor and McCombie are right next to each other, and centre around Park Bar, a bar and food place that hosts sporting events when on, and acts as a social hub. First year I lived in Mackinder, and definitely recommend it as the choice of most sociable, because of how big it is and the overall environment of where you'll be living. Stenton is a joint first, again large and generally always active outside the halls with someone you'll know.
McCombie is very small, but you're next to the other two (literally 2min walk) and will have friends in Mackinder and Stenton, so I wouldn't worry about that - depends entirely on your budget and room preferences. Child's is a quiet hall, so keep that in mind for hosting pre's or socials, however being practically attached to Stenton, will offer you the same benefits of being next to everyone else.
Windsor is the only hall I would outright say do not live in. It's a massive hall and definitely a social one, but after going in there a few times.. I wouldn't get paid to live there.
Bridges is a great hall, I live here right now. It's quite big, around half the size of Mackinder, but it's quite a walk from the others, being on the other side of the lake. This makes it a lot more quiet naturally on a day to day. The security is also centred right in the middle, which makes the mischief people get up to much less frequent here.
You can still have a great social life in Bridges, just as active as the others if you meet people from the halls, outside the halls and in societies - but based purely on personal experience, if you're looking for the most social hall it would be Mackinder or Stenton, as you get the freedom of not being in a quiet hall, while also being in the heart of the student living area with a nice enough kitchen to host pre's and such.
Hope that helps!
Edit: here's the map, so you can refer to my comments:
https://static.reading.ac.uk/content/PDFs/files/Maps/Whiteknights_campus_map.pdf