Just to keep it balanced I’ve used Aztek http://www.aztekintl.com/ and Harbour Shipping in the past when I booked ferries, both very good, not sure if Aztek do the Scottish Ferries, the bloke I mostly work for these days seems to uses Freightlink https://www.freightlink.co.uk/ for the routes he doesn’t have accounts for, so I assume they do a decent job. 
Well I’ve never been stuck waiting for a boat they’ve booked me on. 