If its just grip and not tread depth, then ideally better grip on the front, as that is where you do your most of your breaking and your steering, but if you are talking about more tread depth, then they should be on the rear. A tyre with more tread is less likely to aquaplane in the wet than worn one. Aquaplaning on the front is easier to get back in control than aquaplaning on the rear where you could find your self pointing in the wrong direction very quickly.

More importantly though, they must be fitted onto the same axle.

The usual answer is best grip tyres on the rear.

Sure, it's fine to have two different models of tyres on your vehicle as long as you have matching pairs on the same axle. Very importantly, place the tyres with the best tread on your rear axle. Many people might tell you differently, but this is a recognized tire industry standard. Reason being you what the best tread on the rear of your vehicle to best reduce the risk of loss of control and hydroplaning. Any legitimate tyre retailer would tell you the same.

As long as all the tyres are road legal they will all be ok. Its generally better to put the tyres with more grip at the rear.

First things first, is to check and find out 100% what the tyre size is of your vehicle. I would recommend these 2 sites. Just enter your vehicle registration number and you will automatically get the size and any recommendation of tyres.

http://www.national.co.uk/tyres/

http://www.kwik-fit.com/tyre-search.asp

Furthermore this will make sure you have the right size and correct tyres for your vehicle because if they are incorrect then it is dangerous. And also MAKE SURE you have thread on the tyre because you need grip and without thread means when breaking the car will take longer to stop.

All 4 tyres are 195/50/15 but 2 are make A and 2 make B. Also two are different thread pattern than the other two and have more grip than the other two. Will it be safe to use them? Where do I put the better grip tyres, front or rear wheels?