Whilst I'm not trying to be the Swan and Kingfisher police - I think that the fact that you say 'if I only get one I'll be happy' possibly suggests that you are a little unrealistic in your choices. You are possibly fishing for the fish of a lifetime on those lakes so jumping in at the deep end could be a bit daunting and counter productive on your first visit. I prefer to think that you work your way up to it if you like but then people will try and get lucky so who knows. Fishing Kingfisher is mega exciting and if you are lucky to have them showing it fuels that fire like nothing else, what a buzz.
Mallard holds good numbers of cracking scaley fish that go 30+, it is busy but lots of swims with lots of water to go at. Halfway along the road bank is a fair bet and when I fished it there was a lovely line of weed at around 40 yards that was good to drop just over. Sandmartin is smaller and deeper, the deepest water on the complex, nice corners and margins for stalking, less fish than Mallard but a good introduction to gravel pit fishing - again fish over 30 and possibly a surprise or two. The least pressured lake on the complex but slightly busier than it used to be as people are starting to realise what a decent water it is.
Bluebell is the most circus like through summer, it is a mixed lake plenty of good tench and bream and of course carp to 30+ possibly the best looking lake on the complex.
My best tip would be not to rush in, check all the lakes out and give yourself an hour or so to consider your options. Park at the shop and tell them that you want to check the lakes out before deciding and they will let you do it. Focus on finding fish or likely areas given the conditions first and foremost and let that decide which lake gives you the best situation. Study the weather before you go as well as pictures of the lakes so you arrive knowing where the wind is going to be blowing on each lake and when it may swing to over the course of your session - that way you can read situations and be first to move into the right areas.
I focus on this and getting the basics right such as anti tangle rigs, tried and tested rig styles and components that you trust, stick to what works for you elsewhere, this is not the time to be trying the latest wonder rig. I find clean spots so don't use chods or such like but you have to use what you have complete confidence in.
If you choose the hardest 2 lakes don't be surprised if they turn up in a corner out of the way of the lines and pressure - they are not stupid but if you are stealthy you can have one in no time at all.
Overall I go back to finding the best situation i.e. free swims where you expect them to be or move to over the course of your session, rather than choosing a lake from the outset. Most of all enjoy the experience and learn from it - you are paying good money for the fishing but it is worth it.