I adored Secret bars but I think I have heard that there is no chance they will return as they were so expensive to make.
Also spira - for some reason the shape made the chocolate tasted so much nicer than other bars. As much as I loved them though, I'd hate for them to come back now that cadbury's choc tastes so bad.
Coconut boosts. I may be wrong but I thought that these were the original, one and only boost, and then biscuit was launched and ended up taking over. Maybe it was just that only the coconut ones were available near me though. I absolutely loved them though.
Golden Cups - as a PP said, so much better than a cadbury's caramel.
Also Bitz bars - these were chocolate with crunchy bits of mint or orange - like matchmakers in a bar. I loved the mint one and remember getting a mint bitz easter egg one year!
Those would be my top five but also love lots of the others mentioned here: Fuse, Drifter, Maverick, Frys Five Centres, Mingles, Pyramints - I remember once leaving pyramints in the car on a hot day - when we came back I literally drank/sucked the melty chocolatey, minty goodness from the wrapper.
Cafe near me sells dark chocolate bounty at the till. I always get one as a monthly treat.
Others that I recall:
Top Deck (can still get abroad sometimes),
Strollers (again the same chocolate but tasting better in a different shape and I remember the advert with Black's It's a Wonderful Life), Mars Planets and Mars Delight (or de-lite, maybe?)
Nova bars - (chocolate base with a sort of nutty, granola top)