It's no doubt in my mind that the Dart SLF (as much as i loath them) and the Trident are up there in terms of vehicle types from the past 20 or so years. The influence on the industry they had can clearly be seen today and Dennis were certainly ahead of their time with the Dart which was basically an "extended" Midi-Bus. Today the UK Bus industry is dominated by larger Midi-Buses (E200MMC, Streelite, Versa & Metrocity being the main examples to mind).
The National was probably one of the most revolutionary single deckers, it brought so many new ideas and techniques to the industry that hadn't been done before and are still used today.
The RM (much like it's Boris sibling) was a revolutionary design that grabbed media attention, however neither were exactly successful outside the M25 Bubble. The RM did provide longevity but part of that was due to extensive maintenance including the entire vehicle being rebuilt/refurbished every few years. Compared to other types sales of both were fairly low and really nothing outside London. While RMs did see later life outside London i suspect part of the reason for this was down to Nostalgia and not much more.
With recent vehicles much of the innovations are stuff that are maybe not as noticed by the average joe, stuff like wrap around windscreens which help reduce blind spots and ADL's QRG glass which gives you the stylish appearance of bonded glazing, but without the extensive hours long process to just change a single window. Some people say there's no innovation in the industry today, but i'd disagree. Go on a stock Stagecoach spec Enviro200 followed by a brand new Enviro200MMC and you'll see plenty of improvements. from the automatically dimming interior lights, all in one cab door with bandit screen, USB seatback charging and so on.
A shout out should also go to the B10M and Olympian, purely due to their longevity, popularity and reliability. The B10M in particular (and the "B10 range" in general) is one of the few chassis types that did it all, it could be a light weight coach, Express coach, double decker, local service bus, articulated bus.
Lastly the Solo deserves a mention, like them or loath them it was a very successful product and the only wheel forward low floor bus in the UK to sell in high numbers until that awful WrightBus contraption also known as the Streetlite came along around 10 years ago...
It all depends on how one defines success. If one were to rate success on the basis on the number of buses sold, on both the domestic & international markets combined, the Trident platform in all it's forms (from it's early Dennis days through the tumultuous Transbus period to the present ADL era with the E400MMC/City) is probably the best candidate for the best-selling doubler decker model of the last 20+ years.
In terms of decker chassis sales the "Trident" is actually now the most successful of all time followed by the Atlantean. And with it still in production it's total sales are only going to increase.