I think Sailor Saturn is.

It really depends on what you mean by strong, they all have impeccable strength in different areas. 
For example, 
Saturn: Destruction, Divination to a point
Uranus: Hand to Hand
Neptune: Intuition 
Pluto: A lot: Time, Maturity, Presiceness
So if you take all of this into account, in my honest opinion, the Outers, as a whole, when Saturn isn't using her powers for Destruction are the strongest together. 

If we are including the Destructive powers of the Silver Millennium, Saturn is not even able to appear without the help of the talismans, so in that retrospect, I still don't think I would say she is the strongest, since her power being unleashed requires the Outer Senshi as a whole.

Thats why I always say the Outers don't have a strongest Senshi, and they are, as a Whole, the strongest.