:) If you've got the chance to ride another horse, it's probably a good idea to take it - a different horse can really point out your bad habits.
I like the way my stables arranges horses for group lessons - at the start, the instructor lists the horses that are available to work that slot, and everyone picks who they want to ride. So you can give yourself an extra challenge for that lesson, or make things a bit easier for yourself, and still switch around.
I definitely know what sort of horse I don't want to ride - something from a trekking centre! They're hard-mouthed, full of bad habits and ignore their riders completely, just because even the ones that are given to riders are so used to sloppy work without being corrected for the faults. They know that most of the time they can get away with anything, and then when you toughen up on them they become really unpredictable - a gentle tap from a crop startled the pony I was riding into running down a hill ...
I couldn't say what the 'best' horse I've ridden is, though, because they're all so different. Charlie Brown is an angel and he taught me how to canter but he's not exactly a champion xD I love Mr Darcy, but I also like Fifi ... they're all so different I can't compare them. Fifi has a nicer canter, though, and is easier to do the ... more advanced stuff on. She's more forward, more sensitive, and I actually got a nice trot with lots of impulsion and her head on the vertical :)
If you believe everything you read, better not read.