Was Our Lady the only person assumed into Heaven?

The Holy and Immaculate Conception, assumed body and soul into Heaven.

By Pontifical Definition - yes... 
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The Blessed Virgin is the only one whose assumption is defined as dogma.
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Pius XII defined the Assumption of the Mother of God into Heaven thusly:

44. For which reason, after we have poured forth prayers of supplication again and again to God, and have invoked the light of the Spirit of Truth, for the glory of Almighty God who has lavished his special affection upon the Virgin Mary, for the honor of her Son, the immortal King of the Ages and the Victor over sin and death, for the increase of the glory of that same august Mother, and for the joy and exultation of the entire Church; by the authority of our Lord Jesus Christ, of the Blessed Apostles Peter and Paul, and by our own authority, we pronounce, declare, and define it to be a divinely revealed dogma:

that the Immaculate Mother of God, the ever Virgin Mary, having completed the course of her earthly life, was assumed body and soul into heavenly glory.

45. Hence if anyone, which God forbid, should dare willfully to deny or to call into doubt that which we have defined, let him know that he has fallen away completely from the divine and Catholic Faith. - MUNIFICENTISSIMUS DEUS

Unassuming souls floating in the ether?

Were there others?
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Well for sure Our Lord Jesus Christ ascended into Heaven - that is Gospel and part of the Creed.  There can be no denying that truth either.
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But what about Enoch?   Moses?  Elijah?  Enoch is easy, the book is not in our canonical scriptures, and his story is just 'referred to' in the Epistle of St. Jude.  Moses died however - and St. Michael just hid his body.  But Elijah?  He got taken up somewhere.  It's in the Bible.  Never did a Pope define the assumption of Elijah, although the mystery may be said to foreshadow the assumption of the Virgin, as some of the Fathers speculated.  Interestingly, I found the following reflection upon what St. Gregory Palamas thought about the assumption of Elijah:

He writes: "But neither did he exceed the bounds of the earth's atmosphere; the ascension of each of them was a raising from the earth without being taken from the surroundings of the earth...". From this interpretation it appears that the taking up of Prophet Elijah was in some way a translocation, we could say that it was a kind of death, and that of course in being taken up he did not go beyond the limits of the earth's atmosphere. Therefore only the Panagia (Our Lady) was resurrected and taken up bodily into heaven, and is glorified with her Son, as His mother, in her human nature. - Monachos 

Did you know some of the old Spanish Carmelites believed St. Joseph was assumed into heaven as well?  At least that is what the Spanish fathers told me when I was a postulant in the Southwest province.  I think a few Alcantarine Franciscans believed as much too.  Others, such as the followers of Anna Katherine Emmerich's visions, seem to think that the original paradise of Adam and Eve exists someplace in the ethereal world contained in an immense crystal bubble - and that's where the undefined assumed reside until the end of the world.  Go to the light!  Go to the Light! 
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Thankfully, what we Catholics know for sure is that Our Lady was taken body and soul into heaven - and all heaven and earth rejoices.  Happy feast day to all.

One of those crystal bubbles perhaps?

Art:  Top:  Immaculate Conception, Middle: Hironymous Bosh, Bottom:  Hieronymous Bosch

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NB:  For Catholics in the United States today is a holy day, but not a holy day of obligation in most dioceses since it falls so close to the Sunday obligation.