Chris
The best seat on the market is probably the 'Rutland Deluxe Boat Seat', however it is also the most expensive. I would certainly recommend a seat that rests on the gunwales rather than strapped/clamped to the existing boat seats cause you have better comfort and visibility.
Airflo and Lureflash both do alternative/cheaper seats, and I'm sure many on here will have pros & cons for both. I personally have the Lureflash but it folded under my weight

on Erne in a 4ft swell

. Lureflash kindly replaced it FOC and have since modified the seat to slide like the Rutland seat and upgraded the timber inserts to the aluminium box section to oak for that added strength

2½ seasons now and still going strong...
If I had to do again I would have invested the money in the Rutland/Bob Church style seats...IMO
C