I also adopted a 'grossly obese' cat last year the Rx food the vet gave us he hated and even not eating it he didn't lose weight. We swapped and had a lot more luck with the new one.
Firstly with a cat that big I would say involve your vet, my vet does a 'fat cat' workshop which was really cheap as he spends the whole afternoon just weighing cats and giving out this weeks advice.
Feeding wet will help alot, if you can't for whatever reason, look for a high quality dry with as little carbs as possible.
Don't feed light/diet foods, they are filled with fillers that have no nutritious value and cats tend to store them rather than stop eating because they feel full so gain weight anyway.
Measure food and give them only enough for that feeding - my vet recommends 20-30 cals per lb of weight for maintenance - he recommended 15cals per lb when Scully first started dieting. Don't just fill the bowl up - cats will eat what you give them!
Play with your cat in short bursts so you don't over work them to begin with - 20 mins play time 3 times a day rather than one hour at a time.
Try and make eating play time so the cat has to exercise, our vet recommended that we try putting a shelf on a wall with their food so they had to stand on their hind legs and stretch (like a squirrel) to get food (so about 1.5 feet off the floor I guess - depending on how tall the cat is. That way as they lose weight they don't get flabby.
Cats should not lose too much weight at a time, recommendations say 4lbs a year, Scully lost closer to 15 in 18 months, but it was an extreme case and he was under vet supervision the whole time.
Also you have to be careful that you don't make your cat sick while dieting - fatty liver disease and diabetes are major things to look out for while dieting - so another reason you should include your vet.
Thats all I can think of for now