Well, each has its advantage...

Coach 1 will give you a speedy exit at St Pancras (don't follow the crowds unless you have heavy luggage, head past the buffer stops). It will, however, be full, and consists mostly of tables. It is also the family area (i.e. with tables that fold away to make it more child-friendly, and a baby changing room), so good if you like to hear the song from Frozen over iPad speakers.

Coach 6 has food and drinks, so sitting in 5 (the former smoking carriage - the smell has thankfully gone now!) is a mixed blessing - it really depends on your luck. You have ready access to the buffet car in 6 (you can get there from any carriage anyway) but, well, don't underestimate the capacity of the British travelling public to drink heavily at any time of day. It's not unusual, at weekends, to find the buffet car resembling a very crowded pub, even at 9am. Of course, if you want to join them...

18 is like 1 in terms of layout and family area, and is quite a walk from the exit at St Pancras. However, if you're not in a rush, and the train isn't busy, you can find a lot lighter load down here (if it is, then you'll find it full of tour groups and package bookers). There is also a buffet in coach 13 accessible from 18.

(They do a decent cup of coffee or tea, and are now offering food from Waitrose, but tbh, I normally get something before travelling. I understand that the Paul at the Paris-Nord post-security is closed during renovation work, and that's my normal stop, so I'd get something before security - there's no airline-type restrictions - the list of shops and cafés is here. The post-security waiting area is pretty dreech and crowded, so it's really not worth getting there early!)

4s and 2s, well... I'm 6'1" and would generaly opt for the 2s if travelling with MrsStut, and only 4 if it was a group of people I know. I do find the seats a little low, and the backs a little too inclined - and the fold-down tables are huge! - so it's just passable. In a 4, it's the luck of the draw - you don't want to be sat opposite me, for example The seat backs are high, so you get a much better view in a 4, assuming it's reasonably aligned with the window, although much of the journey is flat farmland...

Do look out for seats with minimal window exposure - the seat map linked to in the wiki:

http://www.eurostar4agents.com/files...at_plan_UK.pdf

should help indicate that.