Fleet is definitely the biggest. Good for Waterloo - direct fast trains. Lots of cafes, OK restaurants, usual WH Smith, Clintons, Works, toy shop, jewellers, post office, etc Lots of charity shops and not very many really nice shops. Sainsburys/Waitrose in town, Morrisons near by. Reasonably good for toddler type classes, swimming pool (driving distance from high street though). Drs, dentists, small hospital etc A few parks.
HW is definitely the prettiest, but really there's not much there. Fish & Chip shop is really, really awful. Costa, One Shop, Post Office. Few 'gift' type shops. Only 10 mins to Fleet though.
Hook is definitely the cheapest. I'm not too fond of it, but it's inbetween size wise.
If I was you, I'd be looking at Winchester. The fast train to London is only an hour, the drive to Southampton is shorter and Winchester has a bit of life to it.