This is probably more about installation procedure than the actual product you are using.
To kill the 'which product' issue, these sealers are all based on silicone but have different formulations. IMO the best ones are marked as High Performance Shower and Bath Sealants and have a 25 year life claim on the tube. This sounds impressive, but can't be proven. This is the widely available Unibond product - shouldn't cost you more than a tenner.
http://www.ciao.co.uk/Unibond_High_Performance _Shower_and_Bathroom_Sealant__6087622
Regarding technique, you really need a gap of between 3-5mm between the bottom of the tile and the flat surface of the shower tray - anything less and you can't easily get a decent amount of bead in there. If this has been leaking slowly for a while, the adhesive/plasterboard/whatever behind there is probably mouldy or pappy and it is really difficult to make it permanent job. You may have to bite the bullet and cut out the bottom row of tiles, clean out the whole area and refix new bottom row, then force the new silicon in there. I don't make the bead protrude - so that the householder can't easily rub it with the CIF cleaner.
Lastly you may have an issue with minor movement between floor and wall when someone stands in the shower - look for this, because no matter how good the sealant, minor movement will stretch the sealant bead and in the end it breaks along the seal line. When sealing a bath, the classic way around this is to fill the bath with water BEFORE filling with sealant - then the sealant is under compression not tension when empty of water. You could try the same technique in the shower tray by loading a few dozen bricks in there to mimic the weight of a person whilst you do the job.