Guess it all depends on the airport and whether your using a nose loader, single or double bridge.
Easiest
757 L2 Door - Just got to make sure your not massively out or you'll hit the engine
A320/21 L1 Door - One of the easiest ones to do in my opinion, nothing spikey to hit.
737-800 L1 Door - Simples
Hardest
737-300/400/500 - Pitots! On the -300 we have 2-3 inches to play with when putting the bridge on, and I'm sure all those bridge drivers know that bridges tend to like moving in feet rather than in inches
777-200 L2 Door - You have to drive 'around' the engine to get it on L2. -300s are easier.
A330 - Not the hardest but your quite close to #1 engine when putting the bridge on L2 door, even harder if your using a double bridge
All aircraft are hard on the nose loaders if the captain doesn't stop where he should
