i've messed around on android quite a bit and keep coming back to this Rom called Dirty Unicorn. If you use their stable release, it is quite solid. even compared to a stock rom, I've noticed it does things just a little better, faster, cleaner. i'm not a hardcore expert on this stuff, you can go to the extreme and use combos of roms and kernels...but the most annoying part is when it comes to updates. updating will either break stuff, or the process will be long and tiresome. what i like about DU is they remove the unnecessary stuff from the stock, which a lot of other roms do also. But they seem to do a better job as far as stability.
It's still not always easy to do a ROM update, but the stable versions last a good long time. I prefer using DU over stock, and i prefer it over any advantage of other rom/kernels because of the reliability. best bang for the buck in my opinion.
to try to answer some of wraith's comments:
if i were choosing a new phone, i'd do it based on the rom i want to run. I prefer DU, so I make sure the phone can run that. It runs on most rootable phones, so that should be no problem.
i used to care about the sdcard, removeable battery stuff. not so much anymore. i feel like the phone's life will be over by the time you really need to consider expanding storage or removing the battery. but if you need that stuff, probably the samsungs are the best bet. if you don't care about that stuff, then i'd go with the nexus line.
battery life...not much you can do about that. android devices should last a day. i'm impressed with the new iphones and how long they can go. i don't think most androids can last as long as iphones, but there are so many models it's hard to say.
regarding updating and the os slowing down over time...this is not always true. but it depends on what is happening. if you are using official channels and non-rooted phones, this is more likley to happen. if you stick to a rom like DU, it will never happen. i have some old nexus phones that are more responsive than the latest quad core stuff now because of the du rom. i hate samsung's OS stuff, i would root and remove it immediately. Samsung's also suffer from weird wifi problems and other odd things...but this is what I've seen on stock samsung phones. I think if you use custom roms, they go away. if you get an android phone, i strongly suggest NOT using any official channels for updates and stuff. don't rely on samsung, or google, or att, or anything. do it yourself. they will mess up for sure somewhere during the life of your phone. that's the headache of android and the fragmented universe.
if you don't want to customize at all, don't get android. get iphone or windows phone. if you can get by without ANY customization...not too many extra apps, no keyboard tweaks, no theming, no rooting, the windows phone is actually rock solid. it will not crash or break and will remain smooth till the end. updates will be headache free, even more so than apple. but again, the only limitation there is that you can't really do much extra things.