i use colleague... but i'm english and i think we're naturally inclined towards being a bit posh and formal, hahaha.
i don't like the word co-worker... it includes the word 'work', for a start, which implies some kind of work actually being done - and who needs reminding there is work to be done, as well as actually having to do it?!... and it sounds a bit... erm... it makes me feel like one of those little green lemmings with miners hats on- a mindless automaton with a small part to play that nobody would really miss if they got blown up randomly.
i found it weird to discover that in Canada, people in lots of jobs don't have 'colleagues' either..... when you're in a store, you'll hear over the intercom/tannoy someone say:
"can we have an associate/team member/team player/*insert other odd word used to make us sound like a happy valued much loved member of a big friendly work community (group hug!!!) here* from the mouldy carrots and cardboard boxes section to the customer help desk please?... customer waiting (this is code for 'haul your arse over here quick-smart', i assume???)"
... i'd hate to work somewhere like this, i'm cynical, but it feels very fake and overly perky (like a bad boob job - no idea why that came into my head... sorry!)... i don't want to feel like i'm being squashed into the 'team player' box (i'm a lousy team player, lol) or like i 'associate' myself (feels like a choose to fratinise with them or something) with the people i work with, or that i associate myself with the company in terms of its values/ethics/ideals/priorities... i want to be around plain old 'colleagues'.