Fighter's Stronghold is worth it for the merchant alone. He's only Apprentice level and has 2000 gold. It also helps early on because of the forge, which boosts your Armorer skill by 25. I also love the taxidermist and putting up my trophies.
Wizard's Tower is good for mages and/or alchemists. The garden in there has everything you'd need for potion-making, plus provides you with the access to the sigil stone glitch and Mace of Doom (fun to mess around with these).
The rest of them are pretty mediocre. Mehrune's Razor is okay, but considering you spend half of your time crawling dungeons as it is, paying money to do it one more time... It is fun though.
Don't waste your money on Spell Tomes. They're usually not much better than what you can buy anyways, and most of them cost so much Magika that they're useless unless you're a full-blown mage.