Unfortunately in the areas where the public have general access before security (i.e. Landside) there are hardly any areas where you can get any good views - actually can't even think of any in T1,2,3. All the Terminals however have some good viewing points once you are through security (i.e. Airside).
In Terminal 1 Domestic Departures, by gate 10 in the Belfast departures area there is a good view of 27R/09L, also up at the 74-90 gates there are good views of that runway.
In the Europier (used by both T1 and T2 flights) down by gates 40-56 gives some good views of the taxiways between both 27L and 27R, and by gate 40 you would see T4 and the holding point for 27L. Down at gate 56 you can see the hangers and the 27R holding point.
In T3 the new gates 30-34 also have a good view of 27R/09L and gate 9 has a really good view along the whole length of 27L/09R, but it's due to be demolished in next couple of months.
In all the terminals you can get good views of the parking stands out of most of the windows between the gates.
Basically you have to be flying somewhere to get access to the very best viewing areas, so it's worth checking in extra early if possible, once you are through security and airside then you can take the bus between terminals to get to the good points via the Flight Connections Centre.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"