my young 18 month is having sloopy poo lately think she has a sensitive digestion what is the best food i can feed her without losing her weight

Honestly, whatever suits her best.  There is no best food as all dogs are individuals and what's amazing for one dog might be awful for another.  Generally speaking, look for a food with a high meat content (meat as first ingredient and actual meat, not 'derivatives') and low or no grain as a good choice to try her on.  Ideally, work out what she's sensitive to if indeed she is reacting to some foods but this may require an exclusion diet to work out.

If it's only recently started then it may be something else - something she's picked up on a walk, or if you've recently changed her food or given her something new (chew, treat etc) that can cause sloppy poo too.

I always keep a few tins of the chicken and rice chappie in, it always works well for my dogs when they have an upset tummy and runny poo's.

You could try a teaspoon of goat's milk yoghourt on her food (goat's milk is said to be closer to bitch's milk than cow's is).  I have found this helpful with both my dogs, one who had very great difficulty (and pain) evacuating his bowels in the morning, and the other had several movements during a walk, which got progressively sloppier.  Both of them have yoghourt on their dinners and both now have very good poos.

I think it is probably best to figure out the cause of the sloppy poo rather than try to find a magic food. 

Has she had a faecal sample sent in by the vet?  Is it clear?  Sometimes things rumble on rather than being a full blown emergency.  Has she been wormed recently? 

If all that has been done, I would try something like Burns Fish and Rice which is low fat (fat is often not great for dogs with dodgy digestive systems) and high fibre to bulk out the poo.  I'd add something like ProKolin to give pre and probiotics and clay to help bind it together.

I'd disagree with the idea of a high meat content diet - this can actually cause even dogs with normal poos to have loose stools sometimes.  Two of mine can't tolerate a diet too high in meat without getting sloppy poos.

It's the bone content of the raw diet that gives nice firm poops.

High meat content in a kibble shouldn't be a problem, because there is fibre content in there as well.  Also, I'm not talking massively high here - somewhere around 20% would be a good start but at the least, having actual meat as the first ingredient is a good starting point.  There are plenty of kibbles now with much higher than 20% meat in that don't cause sloppy poos - quality, not quantity is the crux of it.  In a multi ingredient diet such as a kibble as well, the other ingredients used will have an effect on the digestibility and how the dog responds.

And as gaby says, if you look at a raw diet, around 80% of it is meat but dogs don't have sloppy poo unless they are intolerant to the meat being used (and often that shows in other ways such as itchiness), because the 10ish% bone firms everything up.  My guys are raw fed and it's never an issue.

Yes, but there's no bone in kibble - so if feeding kibble, with something like Orijen, I've known a lot of dogs with soft poops.  (Two of mine can't eat it, the other is fine.)

No but as I said, there's usually another fibre source in it, be that rice, corn or veg.  But I also wasn't talking about very high content foods, again as I said, more around the 20% mark - Orijen is much richer, 85% meat in the original and I can understand that causing a problem with some.  Processing does change how dogs respond to food quite dramatically sometimes.  Personally I don't favour the very high content kibbles because it can be too rich and when I tried Applaws for one of mine (75% meat), she had to have such a tiny amount to stop her piling on the weight that she was absolutely miserable!  But on raw, she's fine and gets a decent meal.