The expansion problem
Dry leaf expands to several times its volume in hot water. If it cannot, the water only reaches the outside of the clump and the tea brews thin — which people then correct by using more leaf or steeping longer, producing something both weak and harsh. Almost every complaint about tea tasting of nothing traces back to this before it traces back to the tea.
What to avoid
Mesh balls and clip-together spoons, which are the most widely sold infusers and the most cramped. Anything smaller than roughly a third of the mug’s volume. Silicone novelty infusers, which are usually worse again. These are fine for a fine-grade black tea that barely expands and poor for everything else.
What works
A basket infuser that sits nearly the full width of the mug, which costs very little and solves the problem outright. Or no infuser: put the leaves loose in the vessel and pour through a strainer on the way out, which is what most of the world does and gives the leaves complete freedom. A simple mesh strainer held over the cup is the most underrated item in tea.
Paper filters and bags
Empty paper bags you fill yourself are convenient for travel and cost a little flavour to the paper. They are considerably better than a cramped metal ball and considerably worse than a loose brew. Nothing here is a moral question — it is a trade between convenience and how much of the leaf you actually taste.