the 20v 1.8t is the easiest conversion (as there's lots of specialists and parts avalible), then again I wouldn't turbo the 2.0 8 or 16v as both would require some substantial head,piston work for reliability.
my fave would be a properly supercharged 8v as the low down torque would be

ok so 160-190bhp (depending on charger,pulley and chip) isn't much but diesel ammounts of torque would be fun without the soot or noise!
(the yanks are running around 220bhp with engine work......16v killer

)