T-shirts are always the easiest to find and buy. I'm picky about necklines (v-neck only, no round or crew necks) and fiber content (100% cotton only), and I still have plenty of choices that look good on my frame.
For me, pants are difficult, because I'm short from waist to hip, but they were a pain even when I was very thin. Fitted skirts are also somewhat awkward for the same reason, so I often choose knits that are cut straight, rather than woven fabrics that are cut curved, because the curves never hit me in the right place.
I think the worst part about plus-sized clothing is that manufacturers assume we all curve in the same places, equally. Some plus-size women carry their extra size in their butts, some in their thighs, some in their stomachs, and some in their back or bust---and any combination of these is possible. The best plus-size stores offer garments in a variety of shapes and styles, to accommodate all those various body types.