I've completed the 3 main campaigns (still have 2 chapters left on the 4th) and have to say that I think I enjoyed the Leon campaign the most, followed closely by the Chris campaign. I didn't enjoy the Jake campaign as much I have to say, even though I did like the melee stuff. All 3 of the campaigns were pretty good I thought, and I enjoyed RE6 more than RE5, but I still don't think it holds a candle to RE4.
I liked the old school RE feeling of the Leon campaign, even if it was slightly wide of the mark. I thought Helena was an interesting character and her presence made the difference. The Chris campaign shocked me because I didn't think Capcom would be able to pull off that sort of action pacing in an RE game, but they actually did a very good job of it. There was a real impetus there that I hadn't been expecting, and the Chris character had a cool arc even if it wasn't the deepest or most original one. I found it hard to relate as much to the Jake and Sherry characters, which is probably why I didn't like that campaign as much as the other two.
One thing I've noticed is that chapter 2 of the fourth campaign is much creepier than the Leon equivalent, simply because you don't have a partner present. The feeling of isolation is a big part of what made RE4 (and the earlier REs for that matter - you were always getting separated from your partner) so atmospheric and I think you lose that with another character standing right beside you. I guess that's the price you play for co-op, and it's a pretty heavy one if you ask me. If RE4 had featured a co-op partner tailing you for the whole game it would have been much poorer for it.