last spring i visited-the somme region
thiepval,ulster tower,lochnager crater,vimy ridge,passendale , ypres etc
all most impressive
there are may small cemetries dotted around the countryside-if you have any relatives who may have lost their lives there,you can research in advance and visit their gravestone
Are you looking at for First World War, Second World War or both? As Beejay says, Thiepval / Somme area is an excellent starting point for WW1. If interested in WW2, try the area of the Normandy Landings and especially the 'Omaha Beach' memorial west of Caen at St-Laurent sur mer.
I have visited the war graves around passendale many times with school trips. i found Vimy ridge, Notre dame dele salle and tyne cot the most movinf. although as the previous answer says there are many small ones. i would always include a visit ti the menien gate and the last post. the last time we went there was a renactment diplay at Zonnebeke which was very interesting. all of the sites were sufficiantly moving to affect a group of cynical 16 year olds