I grew up in Boston and then moved to Nashua. I loved it. It's come a long way and seems to only be improving. Great shops, restaurants, farmers' market, recreation, and local bars. It's a city with a small town feel. It's also within an hour to Boston, the seacoast and an 1-2 hours to some really great skiing. It's NH, so you're going to be really close to a plethora of great hiking, boating, surfing, and mountain biking options.

Manchester is more central and at the junction of all the highways, but I much prefer the vibe of Nashua. Portsmouth is very nice, too, but you are going to find it's a bit further from the mountains and the commute to Boston will be longer with arguably worse traffic.

NH is conceptually not that big (less traffic and people, so getting places is relatively easy). I'd recommend just planting somewhere for a year or so, explore everything and then figure out where to settle.