I had this exact debate with myself about 14 months ago ish, i think!
I test drove them all, i read everything i could and my conclusions at the time were as follows:
Mondeo : I just felt the opinion held on here for them was very overrated. Felt really poor quality inside compared to the fully spec'd accord, indicator stalks like something you'd get in a kinder egg :/ and this sort of thing didnt change in any spec level.
I feel they're hugely overrated on here and couldnt bring myself to drop huge money on them, especially not for an st220 which to me had practically no sense of speed and it'd just annoy the hell out of me and getting crap economy to boot, kind of like the worst of both worlds? mondeo's have a pretty loud and slow as hell diesel option also.
Mazda6 : I've heard some people say the styling inside is a bit tacky but i'm not sure i agree at all. The fully spec'd ones looked pretty nice inside i thought but finding it in this spec was pretty rare. Its standard on the MPS and found in some others if you look carefully. A few things I just didn't like were:
* The chav rear lights.
* The seats look a bit cheap in anything but the top trims which are rare.
Accord :
Command a premium over the other two for sure, as much as a few grand over a mondeo when i wasin the market but the whole feel of things felt a big improvement, especially in the higher spec/trim. Overall it felt like a pretty decent level of luxury without paying out a fortune. The 2.4 variant to me felt a bit like the st220, ie..uneconomical and not quick enough to justify but perhaps not AS bad. The 2.0 seems the daft choice of the bunch. The 2.2 diesel is pretty quiet for a diesel, especially after warmup and pulls v nicely and is allround a very nice drive.
Avensis :
I drove one and while i actually thought they were ok...they did feel a bit "last gen" compared to the other options but the price reflected this too and seem to make them a bit of a bargain. You get the feeling that they'd be very reliable.
Eventually I went for the Accord 2.2 diesel and really have no regrets but a couple of niggles.
The gear changes feel "normal" rather than great. The electrics can act a bit strange at times like sometimes i go to open my drivers window and it doesnt do anything, then 5 seconds later i try again and it does

The engine i'd describe as really nice. Great for overtaking, plenty of power and great economy all things considered. I've been averaging 46-47 mpg combined all winter with aircon / satnav / wipers/ music etc on. Now its getting warmer that'll creep up to 50mpg again or just over. I've owned it just over a year as i say and i've changed 2 front tyres and could do to find out why my left speaker is tempermental! absolutely nothing else at all. The car always feels great and reliable and is a pleasure to drive.