iPhone 3GS. The problems I have are weekly new scratches on the back mostly, though it's still nice enough to hold (not comparing it with newer phones though, otherwise it'd suck), very slow speed for which reason I'll upgrade to a 5 or 5S (or maybe a Galaxy S4!), iOS 7/8 > iOS 6.1.6 and not being able to play like 95% of the games I want to because if the iOS version required is 6.1.6 or less (i.e. a get-able app for me) for the app, then the game simply lags so much that after 1 minute of laggy gameplay the phone quits the app. This problem is most notably in Ninja Slash and Subway Surfers, because I haven't tried any other game I like after those after discovering the slowness.
However, I won't say it is so shit that I should throw it in the garbage. I can get Music, pictures that rival the 4S's camera in high-light (or whatever is the opposite of low-light), small size so easily able to hold it and fits in basically every pocket and I like simplicity over complexity unless literally just too simple, which is certainly not the case here, so yeah. Still though, the phone is so bad compared to even iPhone 4's, which makes me sad.
If you want a new phone though, I'd recommend the Nexus series, Galaxy S4 and if you really don't have a problem with huge sizes then the Note 4 is the best phablet you can get these days. If you wanna stick to smaller phones then you should obviously stick to iPhone (get a 5S if you do!), otherwise if you want neither big nor small then HTC has a few cool but not that cool sets from 2010-2013 that can work well with a size ranging from 4 inches to 5 inches.
To be fair, there really isn't anything "borderline" these days so the only phones you get are either 4 inches or 5 inches, the policy runs somewhat like that. The Sony Xperia series is also amazing (Z3 all the way!) if you don't mind larger phones, and if you can wait a few months then Xperia Z4, One M9 (and One M9 Plus?!) and Galaxy S6 will be your prime choices.
Motorolla isn't that bad either, so you can find a few cheap smartphones that certainly don't lag behind in terms of performance. LG seemingly has stepped up its game with LG G3, but honestly I haven't researched and don't know much about these two companies and how good their sets are. Oppo gets a honourable mention primarily because I love the N3's camera! Aside from that, R5 is amazingly slick and literally 2 mm thinner than the iPhone 6! With that, it has respectable specs but wait... there's no 3.5 mm jack... which is why the x5 MAX by vivo takes the cake for a good thin phone.
My personal opinion remains the iPhone, though, 5S taking the cake for smaller phones overall actually, with the 6 and 6 Plus rivalling other phablets.
Good luck finding a good replacement, and if you don't, stick to your current phone and don't take a risk!