Two favourites from opposite ends of the spectrum - VC10 taught me a lot about engineering and aircraft systems back in the days when everything was mechanical based, whereas now I enjoy the A380 due to the interactivity of the maintenance systems and the amount of information available to diagnose a problem, follow it through a linked AMM and link to an MEL if required..
Not so much an aircraft I didn't like, but the P&W JT9 was a dog of an engine, particularly from the bleed system point of view. Just getting access to anything as it was usually buried below a hot bleed pipe on a turnround followed up by trying to get core cowlings closed when they were based on a baked bean tin design, if the guide arms hadn't come out of the rollers as well!