A few I know of...
The X14 from Bristol to Newport often uses older Volvo B10L, which have very poor seating quality. Compare this to the coaches SC put on the route on their runs! How are they possibly the same route?
Also around Bristol is the 112 (I think) between Highbridge and Weston. It uses deckers, which is all very well, but on a route that has some awkward country sections! (I think it may have been rerouted recently, which would have hopefully solved this.)
As for capacity, I'd like to vote the 179 from Bath to Midsomer Norton. Firstly, it has the dubious honour of being more frequent on Sundays than every other day, for no good reason at all (except I suppose giving Farmborough a better service, but it's not as though it doesn't have a Sunday service). But otherwise, it is perfectly normal for First to put anything on the route that is somewhere between a Solo and a decker!
And then, as much as I don't like to admit it, there is the matter of sending artics up here. If two of them attempt to pass on a certain corner halfway up the Uni drive, one of them has to reverse! It's bad enough in normal buses, but on an artic you sometimes feel that you might slide of the ledge down the slope on to the field below...
Back to capacity, one more I'd like to promote is the 318 from Keynsham to Cribbs Causeway. Courtesy of an awkward interworking arrangement, it regularly makes the journey from Keynsham to Kingswood, which is so poorly utilised it is subsidised, with a decker. It really is a waste; in fact, the entire 318/319/337 operation only needs even a single-deck Volvo around the peaks, and then only for UWE...
Last one now; I'd like to have a dig at one back home, specifically the 7. Arriva destroyed the route, sending it tumbling from Tridents to Darts in just five years. they don't seem to realise that, even now, it tends to run with standing space only for much of the route; instead, they just want a quick buck on a cheap vehicle...