I use Sienna X and I think it's best by a long shot.
Sunless solutions is a nice colour but I find that it often doesn't fade all over but comes off in patches which looks bad.
Fake bake is a love it or hate it brand so you will get clients that really don't like it as it does look fake.
St Tropez can cause problems because people can just go to the shops and then try to do it themselves. I find the colour doesn't last as long as sienna either.
Xen tan fades quickly and looks grubby as it fades.
This is just my personal opinion but i have tried lots and haven't found anything as good as Sienna x. I just use an 8% for people that just want to look sunkissed and 10% for people who want to go darker. If you follow the before and after care properly it seems to last 5-7 days no problem, and I have a couple of clients who manage to get 10 days out of it.
I would advise that it's a little darker than some tans, so an 8% sienna looks like a 10% sunless so that is something you generally have to bear in mind, but sienna has a lovely golden colour, or if you have clients with a more olive skin tone the percentages that end in .5 are more olivey
Sienna x solution is not cheap by a longshot though. They say that you can charge £20-25 for a tan but that depends on your area. Where I am there are lots of people using cheaper brands only charging £15 so I have to charge £15 to get the custom so my profit is less than theres, it depends whether you think you have a strong enough client base to get the £20 price, which it ios really worth