If you use a lot of Google's services, the Android OS is a no-brainer.
Gmail Contacts are synchronized as your phone contacts (so if you add someone's info on your phone, it will be available in your Gmail contacts, and vice versa).
Google Maps, Gmail, Calendar, Talk, Voice, Finance, Listen (for podcasts), Places Directory (quick and easy way to find the nearest bank, fast food, auto-shop, gas station, or whatever), Google Search, and more.
Pretty much the only major Google services I can think of that are not built into the phone are Google Reader and Google Docs.
But if you don't really care about Google apps, then... I suppose I can't offer a useful suggestion.