Having used all of the mobile operating systems that are popular today (Blackberry, Android, iOS, Win Phone 7) I have my list of likes and dislikes about all of them. I'll just rule out Blackberry for you right now since if Blackberry were right for you, you'd already know.
iOS - Runs pretty much exactly as it should with no effort. If you didn't want to change anything, there are no worries. I personally have my iPhone 4 jailbroken and with the apps I use from Cydia it is everything I need from my phone. I have a completely different texting interface, custom lock screens and a bunch of other tweaks to my liking. Not once have I wished for something that couldn't be done. I have in the past used emulators on my phone as well but I don't really care for those sorts of games on my phone.
Win Phone 7 - If you plan to have a phone that works well and doesn't need much tweaking, I would also recommend a Win Phone 7. The LG Quantum is a very solid phone with a great slide out keyboard. I love the interface as it is very refreshing from the 3D icons on a plate and endless scrolling menus that have become the norm. I never found the animations to be absurdly long and the way it interfaces information about contacts is great, linking their facebook, contact info, etc. all in one place.
Android - I have yet to see an android setup that is what I want stock. They are very easy to tweak (generally) and there are tons of options but you have to get it set up yourself.
As for hardware, I think there have been enough suggestions in the thread. The iPhone 4 and LG Quantum feel the most solid in my hand and so I prefer them.
Honestly, right now is a great time to buy a smartphone. People can whine, complain, and play the fanboy all day long but in the end, you can't really go wrong. You will find things you like and don't like about every single phone and every OS. For me, it comes down to this: Want it to work and have the 'popular apps' = iOS, want it for a phone and media device without any customizing = Win Phone 7, want to get inside, mess around, root it and flash it every 5 days = Android.
Good luck with your purchase!