Here are the best trading platforms, in my humble opinion:
1. Tradestation - www.tradestation.com - This is my workhorse. I use it to develop almost every system I trade. It is not perfect, but it is pretty reliable, has a lot of historical data, and is easy to program in (that is why they call it Easy Language!). If you are looking for trading code, you are in luck - the Internet has a ton of Tradestation code in various places, all ready for you to download, modify and test with. Most of the attendees of my Strategy factory workshop (http://kjtradingsystems.com/strategy-workshop.html) use Tradestation, and that course can be free through a program I have with them: http://kjtradingsystems.com/tradestation.html
2. NinjaTrader - www.ninjatrader.com - Many people like this platform, mainly because it is free until you start trading. I personally found it hard to program in, but maybe once you learn it is easier. It also costs $1000 or so, which is a turn off. I do not do any development work with it, but I do use Ninja to help port signals from Tradestation to another broker. It works great for that, and the support is terrific. I am happy with what I use Ninja for.
3. Metatrader 4 - www.metatrader4.com - If Forex is your game, MT4 is a great tool. I use this to run Expert Advisors, do some backtesting etc. I don't create my own EAs (yet!), but I do find that automated EA trading is pretty easy with MT4. I am happy with it.
4. Multicharts - www.multicharts.com - I have tested Multicharts, and it is my back up plan if Tradestation ceases to exist. Most strategies in Tradestation would also run on Multicharts, although performance will be different because of rollover data differences. If I used MC live for trading, it would have a higher ranking.
5. Trade Navigator - www.tradenavigator.com - I have never used this, but some very good trader friends do, and they like it. I have seen it in action a few times, and I like the portfolio testing feature. It may be worth checking out
6. Others - Many people use other software, or languages, like Python, R, Matlab, even Excel. I think you can be successful with these, it just may take more work than with other platforms listed above. But they also have computational power that typical retail platforms just cannot match. So, if the platforms above don't fufill your needs, then give these products a try.
My question for you: which trading platform do you use? Take the 1 question survey: https://www.surveymonkey.com/r/B8RX9SG
By the way, if you have a favorite platform, feel free to mention it in the comments. I'd love to hear from you!