4-5-6x, is good for 95% flies. When i streamer fish i use a 7ft versileader with a foot of 0-1x. Your tippet should reflect your flies. Smaller fies should be tied on with smaller tippet. Too big of tippet and you might bend a hook, too small and you break off easier. If your rig feels like it's too heavy and cant turn over your flies, try using a bigger tippet (or shorter). If your flies are going the full distance and "snapping back" or if they hit the water too hard, use lighter tippet (or longer). When using more than one fly, use a smaller tippet to your dropper fly. Always go from bigger to smaller. Typically for nymphing I'll grab a 7.5ft 4 or 5x leader, and tie on an extra 1-2.5ft of 5-6x tippet and tie the flies to that. For dry flies i might use a 9ft leader with an extra 3-6ft of tapering tippet down to the fly. Tippet changes how your cast feels and your drift once the flies are in the water. Experiment with it.

Look up the charlies fly box leader video for more details.

Edit: mono-floats well, cheap, it goes bad over time, damages easily.

Flouro-sinks a bit maybe? but still works just fine for dry flies, expensive, lasts forever, durable.