Being from the north, if it doesn't get much below 10F, I can ride fairly comfortably for quite a ways in basic windproof/insulated gear. I'd dress like I'm going snowmobiling. I ride in mesh gear through the winter here in FL. Every once in a while, I actually zip a liner into my mesh jacket. Most long-time Floridians are usually wearing parkas, knit caps, and insulated gloves by then, just to walk from the house to the car.
I don't like riding in traffic in 180% humidity when it gets much above 80F though, but up to 95F is OK if I can stay moving while wearing a hydration pack. It helps if the humidity level drops below that of a dripping sponge too. Thankfully, I don't live in a very congested part of FL, so I usually only sit at the occasional traffic light. We don't have much for daily traffic jams or a major rush hour around here. Most riders down here don't wear any gear more involved than a pair of sunglasses and leathery skin, especially during the 11.5 months of hot summer weather. When it gets really hot and humid, I put on SPF15+ sunscreen, a DS helmet that breathes well, armored mesh pants, an Acerbis Cosmo chest/back protector, armored mesh/leather gloves, elbow/forearm armor, offroad boots, a breathable hi-viz t-shirt, a hydration pack, and a coolmax banana-hammock. I still sweat like crazy, but I stay hydrated and the breeze from moving makes it comfortable enough. Florida can be ridden year-round if you can avoid the lightning. I haven't passed out or gotten zapped yet, so the place can't be too inhospitable.
There are a lot of places in this country to choose from that I would find agreeable, as long as it wasn't raining heavily or flashing lightning very often. The rainy season here drives me nuts, but the autumn, winter, and spring riding is pretty agreeable. We just don't get a major show of autumn leaf colors or springtime flowers. It's pretty much just green year-round.
I wouldn't mind riding near the front range of CO most of the year. They get quite a bit of sun. If it gets too cold or snowy, one can go down in elevation to warm up. If it gets too hot, one can go up in elevation. The roads and trails are awesome on a thumper dualsport. Once you get away from populated areas like Denver, CO Springs, or Boulder, there is a lot of wide open.
High-elevation NM and AZ are similar.
I could probably ride most of the year in much of the coastal US that isn't constantly besieged with rain. I lived near Bangor ME for a year, and it only got down to the single digits for a few days at a time here and there throughout the winter. It can rain quite a bit, but the spring, summer, and fall temperatures are very nice. As a kid, I rode minibikes and ATCs year-round up there. If I didn't want to ride salted roads in winter there, I'd stick to riding trails until the roads got clean. A track and ski kit might be fun up there.
Coastal SoCal, from the border to Monterey, has probably the best year-round climate for riding roads. If you stay within 30-45 miles of the coast, the weather is pretty temperate. If you go much further inland and south of LA, summers can get ridiculously hot, while mountains north of LA can get cold and snowy in the winter. It doesn't rain very much in a lot of SoCal too...a few weeks in the winter/spring maybe. The legal/political/economic climate in CA is not to my liking, but I always have a great time when I visit out there.