Best one for me would be doing the Kilchoan <-> Tobermory crossing while on my summer hols in the former a good few years ago. The closest shop beyond that of a corner/village shop was in the latter, so we got rather well acquainted with the Loch class ferry that was in charge of the route during the summer way back when: the Loch Linnhe. Sturdy, reliable and unwavering; weather behaved itself on almost every crossing too. In terms of efficiency, the Corran ferry between Nether Lochaber & Ardgour (which we used on our way to/from Kilchoan) was well up there, especially considering how busy it was both times. Not surprising, considering it's a lifeline service.
Lastly, have had quite a few short, if unremarkable crossings on the Largs - Cumbrae Slip service. One abiding memory from those is the bus service on the Cumbrae side: ruddy busy in the high summer! And the longest crossing I've yet done is the erstwhile Troon <-> Larne service on the HSC Express en-route to/from Western Ireland on another of our summer holidays. Shows how little travelling I've actually by ferry overall if that's my longest crossing lol