Yes the Bwlch-y-Groes from the South is quite good for a "warm up", best to do it with all your camping and cooking equipment if you want to get a little exercise I find.
Now you have whetted my appetite there is one I haven't cycled up yet. Start from the level crossing in lower Harlech - up past the castle - across upper Harlech high street - and keep going and going.... Allegedly the longest steepest continual pitch in the UK. Have to ride that in the next couple of weeks I think.
Suppose I will have to carry all my camping kit, just to make sure I have done it properly - even though it would only be a day ride for me :-)
The climb 35km from Susa in Italy back to the top of Col du Mont Cenis late last May is not the steepest but at 35km long and a with a friend that rides harder than ever for the last 4km makes it stick in the memory from last summer and still makes the legs shudder just at the thought!
Cor that's a good one! Has to be the Ventoux for me, I'm still waiting for the memory of the pain to diminish after 7 years. More recent horrors have included the climb to Les trois chateaux at Husseren in the Vosges. This is not really a monster but the only climb that had me feeling my heartbeat in my hair. Mow Cop and Rainow in the Peak District are toughies but there are several climbs in the Chilterns and Cotswolds that have me regularly cursing. I guess any climb on a bad day can be tough....
Probably not the hardest of climbs in good conditions, but 30 years ago, on a trip north-south across France, I found myself riding up the north side of Mont Aigoual. I didn't actually know I would be going up a mountain, as I only had Michelin road maps which don't show contours etc, and also the day when I left the campsite was very overcast. Yes, that's right, I was carrying a full set of loaded panniers front and rear as well as a bar bag and other stuff on top of the rear carrier. In other words, the bike plus gear probably weighed something in excess of 60 pounds.
Anyway, I set off up this road in total ignorance as to what I would be letting myself in for, which turned out to be something like 10k at about 8-10 %. Plus, as I climbed it got colder and colder, and some 3k from the top I went into the cloud, shortly afterwards passing the snow line. This in early June, and me with only light clothing. The wind picked up, until shortly before the top it was doing upwards of 50 mph - fortunately cross, so I could lean the bike over to stop being blown off. Reaching the top, I found a plateau where the wind by now was the worst I've ever experienced on a bike, but in the middle of the plateau an observatory with an attached cafe loomed out of the cloud, much to my relief. Staggering in, the looks on the faces of those inside as they realised how I had arrived was something to behold, but I quickly found myself gulping down hot chocolate and stuffing my face.
The descent the other side turned out to be one of the most amazing of my life - I quickly came down out of cloud into brilliant sunshine and clear blue skies, with the temperature soaring by the minute as I was now on the Mediterranean side of the watershed. A truly unforgettable day.