Nose, there was no reason to take this thread that didn't specify "only brunswick" and automatically make it out to be that way, bash the brand, and post no helpful choices to fill that spot! Bash them if you want, but at least answer the question.
Boss, it depends on the reaction you are looking for. if you like the rolly reaction you see with the nexus and brunswick in general, then perhaps a versa-max would be good. If you want something that truly bridges the very strong nexus and very weak slinghot, I'd check out maybe a massive damage or a karma solid which will be on the weaker side. Depends on what you don't like about the 4.5. If you're into other brands, the frantic looks to be a very good medium ball that you coudl do a lot with, and everything about the victory road series looks fantastic. Not familiar enough with any of the ebonite brands to suggest a good medium ball other than maybe the hybrid freeze.