Heavy, these are my opinions, and that only.
6.5TCU, accurate, fun, but(for me not enough for hunting deer)yes there are those that do and that's fine but its not for me!
6.5JDJ, very accurate, fun, kills way beyond its means, with the 20" Tender rifle I took a doe this year past 300yds., the 14" is capable of the same, just not in my hands.
6.5X55S.M., Not much I can say about this one. It's history speaks for itself. Its been around a long time.
6.5X284Win., I haven't shot this one much, but it is a proven round.
If I could have but one it would no doubt be the 6.5JDJ weather it be for target or hunting. Not to say its better than any other 6.5, I just like it. But then I haven't met a 6.5 I didn't like.