If you are going to learn a language, make it a powerful, influential language that can be learnt easily and quickly.
Smalltalk remains a pre-eminent language and environment for learning OO concepts.
It is all objects, all the way down. This makes for a really consistent approach to working.
Integers are instances of Class Integer. Strings are a collection of character objects. Classes are singleton instance objects for the class they define.
Control structures work by sending get messages to instances of Class Boolean.
Even anonymous methods (blocks of code, aka blocks) are objects.
Everything is done by sending a message to an object. The syntax can be fitted on a postcard.
The clarity of the concepts and their implementation in Smalltalk mean that you can develop ways of thought which transfer directly into Java, Ruby and C#. I expect it's true for Python, too.
It's so good for making the concepts clear that a major UK University used Smalltalk to train 5,000 people a year in object-oriented computing.
Squeak 5, has just been released. It has gained major performance increases from its new Cog/Spur VM, which features with progressive garbage-collection.
Pharo 4 has a lovely clean-looking desktop theme. The next version, Pharo 5, will be released soon. It will move to using the Cog/Spur VM, it will have about 5,000 classes in the release, and additional packages of classes are readily available from the net via the Configuration Browser tool.
Squeak 5 is performant even on first-gen Raspberry Pis, and is almost 50% faster on the new $5 Raspberry Pi zero. $99 buys you a Raspberry Pi 2, screen and case - running a mature, fully feature-complete IDE.
Leading edge research is being done on co-ordinated, distributed OO systems in Smalltalk (e.g. Naiad and Spoon).
Some of the world's largest corporate databases are run on Smalltalk - including tracking of 60% of the world's shipping containers, and trading systems in the world's largest bank.
Squeak, Pharo, and Amber are all Free, Open-source, open-licenced languages and environments.
Squeak and Pharo provide write-once, run anywhere facilities for MacOS, Windows and Linux. (Possibly RiscOS, too).
Dolphin Smalltalk is targetted firmly at native Windows look-and-feel, and lets you compile closed .exes of your finished work for distribution to end users. Further development of Dolphin by the vendor has stopped, but it is completely functional, and, like all Smalltalks, designed to be massively extensible. (Did I mention that Pharo now has 5,000 classes, compared to Squeak's 3,000? Pharo is a fork of Squeak 3.9)
I learnt it via the Open University, and was immediately productive in Ruby (a copy of the Pickaxe book and the library reference by my side). It helped me enormously with Java, and with Xerox Moo-code.
I have just returned to it to write apps to control manage and distribute responsive, massively multi-platform mobile apps.