Well my Allium Gladiator has put a lot of growth on in the past couple of weeks and I would expect it to be coming into flower next month, but my A. Purple Sensation or A. Sphaerocephalon are only just breaking through.
If you can still buy the bulbs (from a reputable seller) then it's the right time to plant them.
As to which ones are most reliable... it depends which type of allium you want.

It is, in fact, too late to plant the bulbs - they should be planted in autumn. If you find any bulbs offered at sale price, that's the clue - they just weren't sold last year, so don't buy them. You might be able to find growing bulbs at the garden centre, and these can be planted in your garden now. Otherwise, you need to wait till Autumn.

In my experience, the most reliable Allium which never fails to come up is Allium christophii. Allium aflatuense/purple sensation usually returns the second year, and sometimes carries on in some types of soil, but I've noticed they tend not to here, disappearing after 3 years, at which point I buy more and replant in autumn.

Thankyou Urbanite and Bamboo for all the info. I guess I'll have to wait for next year unless I see some growing at the GC.

Make a note for autumn to look for the bulbs. If its any help, Waitrose had potted, growing alliums for sale last week...

J Parkers are selling A.sphaerocephalon as spring planting bulbs for summer flowering. They don't have the large dramatic flower heads but bees love them.
If you're looking for something big and bold why not try agapanthus which are in nurseries now.

Bamboo and Urbanite, that's great news on both counts. My work is not far from Waitrose and i will have a look at Parkers bulbs. Bees are very important to me. I love Agapanthus and wonder if bees feel the same. Time to check it on RHS.

Bees do like agapanthus, but its not a bee magnet like, say, lavender.