The IETs are the strongest in terms of bathroom offering in my experience - some may find the 'space saver' toilets annoying, but I much prefer having the extra capacity - plus two right next to each other massively reduces the chance of any long waiting times. If you don't like them still, as always, the accessible bathrooms are still available! The fit and finish is the highest quality in those, actually feeling like a rather premium option.
What tends to bug me is that toilets always seem to be left untouched during refurbs. The only exception being the new bathrooms fitted to 150s, 156s, etc during the PRM drive. 170 toilets are looking pretty dated these days.
Toilet provision strikes me as rather weak for the 195s and especially 331s, where there can be as few as one bathroom per three carriages, made worse when said bathroom is out of order. Otherwise, I rather like those trains.
Of course there is the 220/221, which has the ventilation system that pipes the smell of the waste storage tanks directly into the main passenger compartment, so everyone can feel like they are in the bathroom. Works out quite well if you can't get to it, due to being crushed in like it's a Tokyo metro line (except you are on it for 4 hours, not 4 minutes...).