The best type rating to buy - one that no one else is buying since you will have a better chance of being hunted down for the job.
The worst rating to get - 737... since every man and his dog is getting one so lots of people trying to grab a few vacancies. What makes Candidate A with low hours and a 737 rating any different from Candidate B with similar hours and a 737 rating?!!! In simple terms - it's not worth the cost!!!