For espalier training, the most suitable apple varieties are those which produce fruit on spurs - which is the short stubby clusters of tiny branches (as pictured below). These spurs produce fruit year after year, and ideally an espaliered apple tree would have spurs along every branch. 

SPUR BEARING VARIETIES

Early ripening

Adore
Divine
Cox's Orange Pippin
Initial
Egremont Russet
Liberty

Mid season ripening

Ariane
Baujade
Freyberg
Golden Delicious
Hetlina
Peasgood Nonsuch

Mid-late season Ripening

Lady in Red
Braeburn
Splendour
Sturmer Pippin
Tydeman's Late Orange.

Late season ripening

Belle de Boskoop
Mother
 

TIP BEARING VARIETIES (not recommended for espalier - except for informal fan shapes)

Blenheim Orange
Fuji
Gala
Granny Smith
Kidd's Orange Red
Oratia Beauty (Gravenstein)
Royal Gala
Sir Prize.