I agree there are few 'weak' fleets.
I know some people around here see space marines as less capable, but then (A) you can always take a Battlefleet Solar fleet, and (B) those things that a strike cruiser does well, it does awesomely well.
What you see as the weakest fleet will also depend on the balance of players in your area. If you have a lot of chaos players with a
Carnage-class fetish (like here) and/or necron players then you will see eldar as proportionately less effective than others might.
DE = good one trick pony fleet. Pretty excellent if you play them like they should be played. Xisor never lost with them. Zhai Morenn neither.
Am currently starting a kabalite fleet since I saw the new jetbikes and went
'want'. Is there a decent exposition on tactics and/or a battle report floating about somewhere?