I've got some expirience with android phones so I can help you out.
My first android device was most certainly hardest - Wildfire S which had good batterylife and specifications as for its times. It's durable but nowdays performance may be poor and when it comes to web browsing or watching films u may have an issue with ARMv6 codecs and HTML5 viewing. I've got LG G2 and ZenFone 5 myself and upgrade to exact 5 inches was best I could do lately. Had S3 mini for some time and it may seem like an anwser. It has exactly 4 inches screen and dual core CPU. Belive me less than 4 inches big screen is only for real masochists. If u want smaller phone then just buy an iPhone or good old Nokia. Heard also good opinions about HTC One S from rom developers on an IRC. From what I see it's decent 4.3 inches with 8 MP camera and good XDA roms support. Devices smaller than 4 inches is some kind of joke that's not produced since times of WFS.
edit mine S3 Mini after 2 years of usage still is in perfect state with tempered glass and cover pop and works as 3G Modem while outside .
Mhm...Wildfire S was one of my first options but the CPU was quite old and it was still a bit large.
As for G2, ZenFone 5, One S and even S3 mini - these are MASSIVE. I need something that's really small and doesn't annoy the f*ck out of my pocket. From your point of view, I may be a masochist, but imo a phone should serve exactly what its name suggests - it should be a phone at the beginning and the end. If it has some additional features, that's cool, but I feel the market has gone the wrong way with "more" in this case.
Besides, none of these are IP67 certified and since I do a lot of hiking, that could be an issue. The fact that Xperia Active is not only waterproof but also works pretty well with wet display is another + for the rainy days on my motorbike.
In any case, surprisingly, I've been lucky - there's been an auction on Allegro lately and I've been able to order one new Xperia Active (luckily shipped from Poland, what the fk is with this all China/Taiwan mailed crap lately). Should arrive somewhere in the middle of the week
Unfortunately no one suggested the Xperia Active so no one is getting the 100k...but since there were 4 guys that were at least a bit helpful - Matt, HelpBot, mkhammer and Koniko, poke me for your 40k cash 'thank you'
ps. Koniko, I can't tell - are you really Polish? If so, it's really nice to see a PL that knows and uses English so well. Almost makes me believe there's still hope for this nation.....almost