I am finishing my first year doing law at Exeter, trust me, pick Lafrowda ensuite! They are in my opinion (and most people tend to agree) the best halls. All of your lectures in the first year are in the forum (alumni auditorium).
Being in Lafrowda ensuite, you are close to the Lemmy (ideal for crawling back after a night where you might have gone a bit overboard at pre-drinks), close to lectures (I've done it in 4 minutes) and as close as you're gonna get to town (20 minutes). You also get 7 flatmates (which if you get on with them are 7 instant friends), freedom to make your own food whenever you like (hello pizza at 3am) and a nice brand new ensuite bedroom.
I would put St Germans'/Rowe House as your second/third choice. If you do choose to go for ensuite, I'd go Exeter Halls (Penny C, Ransom Pickard...). Birks is also nice, but you have a crazy hill to walk up all the time.
Hope that helps !