For _big_ applications, yes, C# would be the best bet BUT....and this is a big 'but', you will have very little productivity for a long time (particularly if you are new to programming!).
To get immediate results and productivity in your day to day work it's hard to go past AutoLISP, that's what it is for! Just start making small routines to automate your tedious tasks, keep building on that base and you will soon have a good library of tools AND a much better understanding of how AutoCAD works (this is key!).
There is very little you can not get done with AutoLISP and there is Terabytes of sample code out there (just look at the size of the AutoLISP board here at The Swamp!)
I'm assuming you are relatively new to programming so there are some basics you need to learn such as:
- what are variables and scope
- data structures
- conditional branching
- loops and recursion
- functions
- how to use libraries
- how to structure an application or library
- etc. etc...
Lisp as a language is one of the simplest languages to learn, it is elegant in that it has very little syntax ('everything' is a list!) and writing code is done by writing to a text file in a simple text editor. This makes learning the above a lot easier to do as you don't have the extra cognitive load of learning how to use IDE's, wiring up correct frameworks/API's, OOP programming paradigms et al.
In earlier days there was VB/A which also had a low barrier of entry into the programming/automation world and was a lot easier to 'level up' from (to say C#) but it is not really viable these days.
A few months/years learning programming with lisp first would not be altogether wasted even if your goal is to learn a more 'imperative' style language and on top of that you'll be a lot more productive a lot quicker!
Programming is a very long term mission/goal/obsession so if you are in a hurry to get stuff done AND learn how to write programs then I'd dive into AutoLISP. If you go the C# route be prepared for a steep learning curve and to learn next to nothing about programming AutoCAD 'productively' for many weeks->months (you need to learn the language and tools first) but the choice is ultimately yours
Good luck!