Assembler
Assembler is the most obvious language that you'll consider using as you probably won't need to buy any other tools to use it. More than likely the manufacturer will provide an assembler for the chip and you won't need any books as all the instructions are in the datasheet and you just start coding.
This route to programming is very easy but you may be setting yourself up for problems later on as:
It is trivial to write short programs. It becomes progressively more difficult to write large programs. It seems to be the best option as it gives the fastest code. It seems to be the best option as it gives the smallest code.
Assembler: Fast & small code
There is no doubt that assembler gives the fastest and most optimized code (your brain is better at optimization than any compiler!) but assembler is difficult – typically you'll spend ten times as long writing assembler as you would writing in a high level language.
Assembler / Compiler Trade off
This is the trade off; to write the fastest most optimized code or to get the task solved more quickly.
Another problem with assembler is that to do even the most trivial task you have to think about every aspect of the code and all implications on registers and register flags.
Even making a microcontroller perform the most trivial task is difficult e.g. for making a loop in assembler you need to think about which register to use and which instructions all the while thinking about how those registers should not interact with the loop register/other registers etc.
Assembler: Problem - changing the target
Another difficulty is when you change from one microcontroller to another (even in the same device family) the assembler instructions may be changed e.g. more instructions to improve microcontroller performance. So you will have to learn an entirely different instruction set when moving either to a new target microcontroller or moving to a different device within the same family i.e. code re-use is not possible unless you stay with one microcontroller (or devices with a similar internal architecture).
High level languages
HLL: Retarget
Retargeting code to another microcontroller is easier since the HLL will know the details of the new target i.e. instruction set, fuses etc. All you need to worry about is the specific differences between the different microcontrollers (in the same family this will be setting up the internal peripherals).
The important point is that the HLL takes care of the assembler code needed to do the job.
HLL: Easy to understand.
The most useful aspect of a high level language (HLL) is that the language is written in a form you can easily understand – there are no cryptic assembler commands that you have to remember and most commands are made up of several machine code instructions – saving you coding effort (often there are built in libraries of code e.g. LCD driver, Serial port driver, I2C driver etc
So the HLL makes it easy to write code as it generates the correct assembler for the target microcontroller.
HLL : Whitespace
You can also make use of white space (areas of no code) to separate out the various operations within the program – typically assembler code is just one great big list that is really very difficult to read – I know there are comments but you need to comment almost every line so that someone else can
understand the code.
HLL: Task splitting
One of the best features of a HLL is that you can split tasks into separate functions that you can concentrate on them individually (as the HLL takes care of local variables etc.). For assembler even when using a call instruction you have to take care of preserving the register state – in the HLL it's all done
for you.
HLL: Code re-use
Once you learn the HLL you will find it easy to read code written by other people and you will be able to re-use code that you have already written whereas with assembler you will constantly need to analyze the code to see if it fits in with your new functions.
The only decision then is which high level language? There are really three contenders BASIC, C and Pascal – these are the most popular languages and for popular microcontrollers there will be an HLL compiler for each one. I'll just list the advantages and disadvantages of each
BASIC Advantages
Very easy to learn and use. A BASIC compiler will produce code that runs fast as a C compiler. Many in built functions (depending on compiler). Very popular – large user base with many example programs.
BASIC Disadvantages

Non standard language. If using an interpreted HLL will run very slowly.
Note: Because the language is not standardized it will be difficult to move code to a new processor target type.
Pascal Advantages

Easy to learn and use. A Pascal compiler will produce code that runs fast as a C compiler. Many in built functions (depending on compiler).
Pascal Disadvantages

Not as popular as C – so not as many compilers. A bit wordy – it was originally intended as a teaching language. Not as flexible as C.
C Advantages

Compiled language - always runs fast. Standardized language (ANSI)- easier to port to different compilers / target devices. Many compilers available. Many in built functions (depending on compiler). Very popular – large user base with many example programs. Used in many different industries. Usable at the hardware level as well as higher abstraction levels (although C++ is better for very abstracted programming models).
C Disadvantages

Hard to learn at first. Strong type checking means you spend time pleasing the compiler (although this protects you from making errors).