Karl was extremely critical of seemingly every other socialist thinker of his time, save for his friend Friedrich Engels. He would call Karl Kautsky, who deeply admired and arguably idolized Marx, a "small-minded mediocrity" in a letter to his wife. He wrote many polemics on many of the philosophers and socialists during his time. Interestingly enough, Engels would enjoy Kautsky and his company to have drinks, and Engels would praise philosophers like Max Stirner saying "It is equally from egoism (Stirner's philosophy) that we are communists." while Marx devoted a chapter of his book The German Ideology to denouncing Stirner and slandered him naming him "Saint Max". Marx was also regarded by many as ill-tempered, as expressed by Kautsky before meeting him. My question is, aside from Engels, was there anyone alive during Marx's time who he admired or respected for their philosophy or politics?
Marx liked plenty of people. However, Marx wasn't in the business of being an academic or a respected figure; Marx was a radical. He was interested in criticizing everyone and everything regardless of the personal consequences (evidenced by the fact that he was exiled for his writings twice). Marx says as much in an letter sent to the editor of the magazine Marx wrote for, Arnold Ruge.
Now philosophy has become mundane, and the most striking proof of this is that philosophical consciousness itself has been drawn into the torment of the struggle, not only externally but also internally. But, if constructing the future and settling everything for all times are not our affair, it is all the more clear what we have to accomplish at present: I am referring to ruthless criticism of all that exists, ruthless both in the sense of not being afraid of the results it arrives at and in the sense of being just as little afraid of conflict with the powers that be.
While he was incredibly critical of them, Marx had a some degree of respect and admiration for the work of Adam Smith, Ricardo, Stirner, Feuerbach, Proudhon, Hegel, and others. Marx was friendly with August Bebel as well, although I can't remember Marx ever actually writing anything explicitly positive about his work so I can't say much for how well Marx actually thought of Bebel's philosophy and whatnot.
Marx worked together with Jules Guesde and Paul Lafargue (with the help of Engles) to produce The Programme of the Parti Ouvrier in 1880. However, they would have a falling out not long after the writing of the Programme, primarliy over differing opinions on the value of reformist struggles (Marx at least gave them some consideration; Guesde and Lafargue would have none of it).
“Discounting the possibility of obtaining these reforms from the bourgeoisie, Guesde regarded them not as a practical programme of struggle, but simply ... as bait with which to lure the workers from Radicalism.”
Marx, on the other hand, considered reformist struggles to at least be of some worth, although he would also write in Value, Price, & Profit:
At the same time, and quite apart from the general servitude involved in the wages system, the working class ought not to exaggerate to themselves the ultimate working of these everyday struggles. They ought not to forget that they are fighting with effects, but not with the causes of those effects; that they are retarding the downward movement, but not changing its direction; that they are applying palliatives, not curing the malady. They ought, therefore, not to be exclusively absorbed in these unavoidable guerilla fights incessantly springing up from the never ceasing encroachments of capital or changes of the market. They ought to understand that, with all the miseries it imposes upon them, the present system simultaneously engenders the material conditions and the social forms necessary for an economical reconstruction of society. Instead of the conservative motto: “A fair day's wage for a fair day's work!” they ought to inscribe on their banner the revolutionary watchword: “Abolition of the wages system!"
While this issue would eventually divide the two men, Marx's willingness to work with Guesde in the first place indicates that Marx, at the very least, had some level of respect for him.
Marx was probably most vocally supportive of Darwin. Marx considered Darwin's work to have provided a scientific basis for his philosophy. The accuracy of that belief, obviously, is fairly controversial.
Darwin’s work is most important and suits my purpose in that it provides a basis in natural science for the historical class struggle… Despite all shortcomings, it is here that, for the first time, “teleology” in natural science is not only dealt a mortal blow but its rational meaning is empirically explained.
With that said, Marx's view of Darwin's theories would change over time. While always supportive of them, Marx would give them less importance later in life, primarily due to the piss poor efforts to merge Darwin's and Marx's work into one, singular whole, something that both Marx and Darwin recognized as impossible.
Mr. Lange, you see, has made a great discovery. All history may be subsumed in one single great natural law. This natural law is the phrase (the Darwinian expression becomes, in this application, just a phrase) 'struggle for life', and the content of this phrase is the Malthusian law of population, or rather over-population. Thus, instead of analyzing this 'struggle for life' as it manifests itself historically in various forms of society, all that need be done is to transpose every given struggle into the phrase 'struggle for life', and then this phrase into the Malthusian 'population fantasy'. It must be admitted that this is a very rewarding method - for stilted, mock-scientific, highfaluting ignorance and intellectual laziness.
This article from the International Socialist Review provides a pretty good, if very biased, explanation of the relationship between Marx and Darwin. Additionally, Anton Pannekoek's Marxism & Darwinism provides a good rundown of the link between Darwin's and Marx's work.