For trout up to about 4-5 pounds, I use 2 pound. For steelhead, I use a 6 pound base with 4 pound tippet. 4 pound is sor of a compromise if I ecpect to run into big fish.
This is spinning gear of course.
For a fly rod, I usually have my 7'6" Orvis Brook trout rod. 4wt, technically, but I re-eyed it. It will cast a 3wtfwd, 4wtfwd and 4 DT(sink or float) or 5DT (float) also. Tippet depends on the fly...usually 14-24 sized hooks for drys, or down to size ten streamers and nymphs. I also have a 6wt Ross pack rod that is OK for packing. A bit heavy, but it works for #8 streamers, large nymphs and small bass bugs. Often, I will blend this with a Childe Spinning reel for trolling in the boat or casting. A really good casting rod for lakes at 8'6"...it will cast a spinner a bloody "mile." with 4 pound. I carry an extra tip eye in case it gets worn. I have several others but I don't carry those in since I broke a couple over the years.
For bushwacking and a hard climb into some smaller water, I would siggest the 2# with some #10 and 12 hooks and maybe 5 or 6 size 16 hooks for catching small bugs and baiting your own hook. A few small bobbers works great when coupled with a 5' 2piece UL rod. Fast action graphite works pretty well.
Here is a picture of a 3-3/4 - 4 pound brook trout I caught in the rain a couple years ago on 2 pound test with a streamer. (Note the fresh hook was added after clipping the streamer off and first 20' of line. For reference,the cup measures about 3-1/2" in diameter, never bothered to really scale it. He was supper for two and a dog.):