Yeah, we've beaten each others score on Pray so many times it's a bit mad. Other than that, i usually try a fair bit of what i have in my playlist. If you want something somewhat challenging, the original version of JENOVA with iron mode on is a good challenge. Forces from Berserk is a decent one to go for as well, Though the god hand mix is actually slower, i found that a little weird.
To be fair, i have so many songs to try out, i really don't know what to recommend, hah. Depends on how much of a challenge you like as well, since i do go for more ridiculous ones sometimes(try Ocarina of Times Final Battle music)