there isnt a "best wormer" as such. Best thing is to get the vet to get a faecal egg count then devise a worming plan for your horse - worms can get resistant to wormers if you use ones with the same classes of drugs all the time. The egg count will make sure you treat what your horse already has - you may not have to worm your horse quite as often if the parasitic birden isnt too high.
If you want something that covers the most types of worms then you will want something like the Panacur 5 day guard, Equimax or Equest Pramox, but as mentioned above you will need a proper worming plan to make sure you are alternating chemicals each year and covering for the right type of worms at the right time. If you get them from a shop there should be someone there qualified to give advice on the best product for your horse!
The best WORMING you could do and this goes to help the entire horse population is worm counting. This slows the rate of resistance and therefore makes it more effective. Blood test for tape once per year only.