ASB is a nice choice and is quite winter hardy. Some other good blues are Blaumiese (lacecap), Blue Bird (lacecap), Blue Deckle (lacecap), Blue Prince, Enziandom, Gen. Vocomtesse de Vibraye, Hamburg, Marecahl Foch and Nightingale. Check the Internet for pictures.

Bottom line: are you looking for strong rebloom, or just size with a good old summer bloom only?

For size and reliability, stick with Nikko. It gets large enough and sets enough buds up and down the plant to bloom reliably for most zone-6'ers. It can also act like a rebloomer as its stems grow at different rates.

Dooley blooms extremely early (May) but can suffer a little bit more from dieback than other remontants.

Penny Mac is a good one because it gets a little larger than ES, and its rebloom is touch-and-go but not impossible.

ASB's rebloom is also touchy or non-existant for some as far as I know.

David Ramsey is another good choice. It seems a bit more hardy than ES for me, is cheaper (you can buy bigger).

^ Now all of the above don't really have much color advantage over another. The blues are mainly pastel in nature and are hard to get deep.

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The real advantage does seem to come from the older varieties, such as Enziandom (Gentian Dome) as Luis mentioned. People have widely labeled this one as the best deep blue.

I'm not certain of the one I have (got it from my Grandma's funeral arrangement in 2011), but I believe it's Brestenburg, and is producing some extremely deep blue blooms from stems at crown level with no winter protection.

thank you all for the suggestions... When the Endless Summer craze hit a few years back, I got rid of quite a few Nikkos, thinking I would have constant bloom from ES's. I cant even get one good flush of blooms on the ES's... I've tried them in sun, part sun, shade, etc. and its basically hit and miss... some years are decent and some years nothing.. I see some beautiful, big old 5ft Nikkos (I'm presuming) around town that bloom year after year on very sturdy, upright, bushy plants. The ES's seem to be somewhat weak, "leggy" growers, often just flopping over, and not getting much larger than 3ft tall. I missed my Nikkos so much that when I stumbled upon a few- by pure luck- I bought 3 nice ones in 7 gallons. I was shocked to find them in Cleveland. I thought I would experiment w/ a few various remontant or very cold hardy cultivars and see if there was much variance between them, but I've heard that there isnt much-- just thought I would check w/ the experts on the forum here. I also saw some ASB's at the same place I found the Nikkos so, I guess I will try them as well.. Hardiness doesnt seem to be an issue, I usually mulch w/a bit of leaf compost, but I cant recall significant dieback this close to Lake Erie. I would love to find a very deep blue like the Enziadom, or some of the other older "standbys" but the problem is locating them, in that every nursery is saturated with Endless Summer. Anyone know a source for Enziandom?

You can go to the website for many mail order places and find it: hydrangeasplus.com, nantuckethydrangea.com, joycreek.com

Thank you for the links, luis! There is certainly quite a bit to choose from. I'm curious as to the hardiness of Enziandom in zone 6? any thoughts?