I've found the best or most popular contests are the simple ones that don't require massive amounts of time to enter. Not everyone has the time to spend on writing huge posts (some do, and that's great too, there's a place on the blockchain for original content of all types), so contests that are quick to enter are always popular.

As for the prize types, I think diversity is the key. SBI shares are great, Steem Monsters are also popular, but there also Incinboost shares (similar to SBI) going around now, I've seen prizes of SP Delegations for a period of time, or sometimes even a physical prize is cool. I was very excited last year to win a physical silver coin in a contest run by @o07.