I have VDL, Niobe, MJC, Huvi, Gravetye Beauty and Ruutel.

Gravetye Beauty, a Texensis, is a really beautiful red red for me (not floriferous, though).

MJC and Ruutel for me are also close to red.

MJC has bright red smaller viticella flowers.

Ruutel has bigger darker red flowers.

Ditto to what carefree said about Gravetye Beauty-- a nice red but not overly floriferous for me either.

My texensis is a nice red colored clematis but you have to be careful of the source or you may end up with a pinkish colored one.

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I vote for Niobe, a beautiful, dark red velvet looking and a prolific bloomer. Allanah is second with its large six sepalled flowers that are a deep red, but don't look quite as dark or have that velvet quality. Both are blooming now and Allanah is huge and has about 20 very large blooms on it with more to come. IN fact, it is growing right out of its trellis and flopping over. Niobe is just starting and the flowers not quite so large as I moved her over a year ago and she is still recuperating.

Allanah in my experience is more of a fuschia colored clematis than a red. In fact, I purchased it several years ago from Chalk Hill based on the picture they had at the time on the website and it looked nothing like the red they showed. They now have a better picture on the site showing how it looks in real life. Here are a few pictures that are very representative of how it looks in my garden.
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Niobe is a nice colored clematis and its flowers change colors as they age. They tend to open almost a black red and fade somewhat to a more true red.

Newer flower.
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Older flowers
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I have Niobe, Ville de Lyon, Warsaw Nike, and Ruutel. In my garden, Niobe is an incredible deep, deep red. It opens almost black-red, then progresses to a deep true blood red, and fades to a light burgundy.

Ruutel is pretty red when it opens, but not a clear red like Niobe. It definitely has a noticeable purply cast that blues more and more as the flower ages. Very pretty though.

Ville de Lyon could only be called a red in the old-fashioned sense of really bright deep pink - carmine or fuschia or something.

Warsaw Nike could in no way be seen as a red by any stretch of the imagination in my garden. It is hard to imagine differing conditions getting it there. It is my fave clem of all though (other than Niobe and Vesuvius Galore). Not sure how to describe the color, but the word Midnight (it is aka Midnight Showers) is apt. A deep velvety purple that carries both intense blue and red casts - it just glows like few other flowers of any type.

I can not stress how much I absolutely LOVE Clematis "Mme. Julia Correvon"..I oogled over picture of her from so many clematis fanatics on the forums for two years before I finally got her...I planted her 2 years ago and she was Stupendous her second year!!..Almost completely enveloping my climbing rosa "Eden"..Madame "Eden" best get off her tush and grow grow grow to handle MJC!!..And she is..Also, one that I don't see people talking about is Clematis "Barbara Harrington"..she is a lovely vigorous pruning group 3 red!!..
I have had "Niobe" for 4 years and she finally did her thing this year...what a slow slow Clematis she has been "for me" to get established in my gardens..BUT I do have her in partial shade to ensure she doesn't fade..I had considered relocating her but after her performance this year..she is staying where she is!!..
Another Fabulous Clematis I have growing is "Voluceau"..what a delight this one is..but she isn't really red red red...She is more Fuscia purplish to me..but what a stunner and grand performer she is!!...you and I seem to love the same colours in our gardens...Last year I went on this fad of planting Purples and Reds together in my gardens and my husband loved it so much he requested I do more of that combo....Jeanne

Barbara Harrington has been a dog for me. Niobe and Ruutel have also been slow but not as slow as Barbara.

Ville de Lyon has also been a really slow. All were from gallon sized pots and should not be so slow. I'd be thrilled if they were as big as Miguel's pic of Niobe but they aren't even close.

I saw a sea of Ruutel's blooming at Lowe's the other day and am glad I have one. I would not call it red but it is gorgeous.

It's amazing how one clematis can bloom so much for one person and not for the other. Just when I get to the point of deciding, okay, guess it will be Niobe or guess is will be Barbara Harrington, I move on to the next post and that clematis isn't doing nearly as well for that particular poster. Makes it hard to decide which one to purchase. I may just make it easier on myself and purchase one from my local garden center, they had a bunch of Niobe's in stock and if they still have some left, I think I'll buy one this weekend. They're pretty big and have a good root system. I'll also avoid the high shipping costs.

Thank you all, you've been extremely helpful!

Linda

I have to ditto Jeanne re: Julia Correvon. It was my only clem to give me first year blooms and I adored it. Of course, red is my favorite color, so I think I'll always be partial to her. :0)

Barbara Harrington is not red. It is pink, at least in my garden.

Rosemoor, Rouge Cardinal, Niobe, Mdme Julia Correvon are my choices in red. Less flowers is Texensis.

Crimson King is pink compared to Rosemoor and Rouge Cardinal.

Looking for tons of blooms then Julia and Rosemoor get my vote.

Dana

Dana, don't know how long texensis has been in your garden or if it might just have less blooms in your colder zone than in the warmer zones, but texensis is going gangbusters this year in my garden and has as many blooms as Rosemoor and Julia Correvon ever had for me--perhaps since it is a native to Texas, it likes the hot dry weather we have had this season.

My Niobes have also been slow growers. Like Jeanne's, mine were purchased as large plants from local nurseries. I think it's a good plan to get bigger ones on this cultivar. Another downside, in my garden, is relatively poor rebloom. But it more than makes up for its minuses with its incredible color and generous initial bloom, IMO.

Wish texensis would do gangbusters here. It is such a beautiful shape!

Hot and humid is what we have. Oh yeah, June had 6 days without rain. Princess Diana loves it. Never seen so many blooms. Too bad the black raspberry canes took over. It is a hard place to get a good picture from.

Dana

Dana, send your excess rain south. We need it!