I've spent a worrying amount of time today working this out - and discovering that because of a few kiddie coasters I always forget, it's not what I thought it was...
My 25th was apparently Nemesis Inferno, when I'd always thought it was Zipper Dipper. Turns out I couldn't count... bit disappointed, actually - I liked having it as a silly ride.
At least #50 makes up for it - as that's Nessie at Rhyl Ocean Beach, as a result of that fun day in September 2005 when I trekked over there with Rob and Furie, in search of my 50th. I went on a lot of rubbish that day xD
#75 meanwhile was Green Scream at Adventure Island. Meh.
Finally, #100, which I finally reached this Summer was Ghost Bomber at Movie Park, Germany. I admit, I'd have preferred a major coaster for my 100th, particularly with it happening on such an awesome trip, but there was no point faffing with ride order etc. just to make it so.
I'm still only on 97 UK coasters, if we're being fussy, so I'm intrigued to know what rubbish I can find to be my 100th on home soil xD