Favorite DotA 2 character

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Haven't played any new PC games in a very long time but a few days ago I installed DotA 2 and it's pretty fun. I tried a few heroes but I somehow stuck with Viper. It's just very strong and easy to play and fits the style I'm playing. So now I'm using only Viper and trying to master it. So who's your favorite DotA 2 character and which one you hate the most? Do you tend to play just 1 character if possible or you mix it up or do you change it depending on the enemy picks?