Since I have no smartphone (its on the Xmas list), I can't tell you whats best, although my wife is happy with the Nokia Lumia series running Windows 8 (one for home, one for work).
However, I do work part time in a dept which sells a lot of tablets, so my best advice is go into a store and have a play with Iphones, Android and Windows 8, and have an open mind.
When people ask about tablets, the first question we ask is 'what phone' (and other equipment) do you have? If they have an Android phone, then thats a good start, or if Apple, then look at the Ipad - trying to use two different OS's is a pain, and the phone etc should be easy to use. I have to admit we sell more Ipad's than anything else (they are just really easy to use) , and a lot of Samsungs came back last year as Ipad's were in short supply (an Apple trait), so people bought Samsung Tab's as presents instead, because 'they are really similar'. Since people had wanted an Ipad, back came the other tablets after Christmas, and bought Ipads instead.
I suspect you've got used to the Samsung - which is perfectly fine, and perhaps an S5 would be a good step up. However, operating systems are very personal things, and once you've got used to one, its often very difficult to go to something else. I've got an Ipod Touch, so an Iphone would be perfect for me, and I have to admit I find find Android a bit fiddly. However if your used to Android, its just going to feel...different to use something else. Same with Windows 8.
Go and try them all out. See which one feels right to you, and then go from there. There is no best or worst system, just what works for you. The one thing your not going to lack is choice...