Is Christianity the most optimistic of all the great religions?

Although this really depends a lot on what you take optimism to be, I don't think that's really a defensible contention at all.

Several religions believe in an attainable paradise that can be reached if certain conditions are met, so Christianity is certainly not unique there. But unlike those religions, Christianity assumes that humans are completely debased and depraved. Whatever our faculties ("made in God's image"), Christianity assures us there is no hope for any of us without the favorable whim's of a monarchical father-figure. And that whim is favorable contingent on believing something that is not well supported by any decent evidence. You cannot sincerely believe something that doesn't seem to be true or otherwise makes no sense. So it might be "easy" to gain the Christian god's favor if you're already disposed to the religion, but it isn't possible if you actually think critically about it. You can no more choose to believe the Christian myths are real if everything says they are false than you can simply choose to believe red is blue. That doesn't seem optimistic to me.

In a somewhat altered form, the same pattern for attaining paradise exists in Islam as well (that is, the ultimate deciding factor is the whim of Allah - what Christians call "grace"), but Islam lacks the same Augustinian belief in "original sin" or the inherently debased, Hobbesian character of man. So I'd say that within the basic Abrahamic worldview, Islam is already a more "optimistic" religion than Christianity.

This is not to say that the other major religions are significantly better. Buddhism, for example, though it has a positivistic bent not reliant on moralizing (i.e. Buddhists speak of objective "suffering" and not the supernatural concept "sin") and it teaches that the means to attaining serenity and harmony are available to every man, still has an unhealthy dose of blame-the-victim-ism injected into it in the form of "karma." Because if bad **** happens to you, like social exploitation or genocide, that's totally because you did something bad to someone somewhere at some point. (And yes, I know that some people regard bad things happening as "tests" of one's character, but at that point the whole karma hypothesis is totally unfalsifiable.)

Ultimately, perhaps it is Taoism, which lacks explicit concepts of sin and karma (though religious syncretism is an ongoing process), that is the most optimistic of the major religions. It's hard to say, because Taoism is a bit more like certain Presocratic attempts to interpret and explain nature than it is explicitly supernatural doctrines that grew out of stories about magicians. What it does suggest is that harmony with one's environment is ideal and attainable. Of course, it can also be viewed as a "do nothing" philosophy, but that generally stems from a bad reading of what balance requires.

Yes and no. The fundamental tenants of Christianity being forgiveness, turn the other cheek, give your money to poor people, yay for the meek are about as high as you get for human virtues. But if you look at the old testament you get into some real nastiness, and the entire trend of 'If you don't believe what I believe you will suffer forever" isn't really that optimistic.

It i a religion for the hopeless. A religion for those who want to put themselves above others, those who fear death, and those who fear for their afterlife. Its rather pathetic when you think of it.