For steelhead and Pacific Salmon 99% or better of my fishing is done with a 2-hander simply because it is more efficient to cast 50 and more feet with a 2-hander and it provides far more line control than the single-hand rod. Not to mention the ability to cast to where the fish are when there is nearly no backcast room. My favorite rods for this are a 16 ft 11 weight fast action rod in winter, and a 13 ft 8/9 fast action rod in summer. The other 1% of the time, I use an 11 ft 8/9 fast action single-hand rod and I am nearly always simply single-hand spey casting with it. This rod is used on small streams that have virtually no backcast room that have good runs of steelhead or coho.
For trout and bass, I use only single-hand rods. 7 1/2 ft 4 weight on small streams or 9 ft 4 weight with small flies on medium and large rivers; 10 ft 6 weight for general trout fishing on large or medium sizw rivers and most lakes; and 10 ft. eweight or the 11 ft 8/9 weight for bass and large trout streamer fishing.
However, since I spend 90% of my time fishing for steelhead, I rarely use my single-hand rods anymore.