simonsunnyboy

And software support for it is probably non-existant. I doubt any game or demo will make any use of such addon.

There is a free and actively developed ANSI C compiler available. Try AHCC from http://members.chello.nl/h.robbers/
It is compatible with Pure C, ANSI compliant and has full support for TOS and all its subsystems.

The "best" language is a matter of personal taste. For a plain beginner, GFABASIC is a good choice. If you already have experience with C, go for C.

You have full access to all chip registers. They are accessed with memory mapped I/O ports as follows:

===========#==#=======#===============================================#=====
----------------------|YM2149/AY-3-8910 Sound Chip				    |-----
===========#==#=======#===============================================#=====
$FFFF8800.B|R-|PSG_SEL|Read Data									  |
		   |-W|	   |Register Select							    |
$FFFF8802.B|rW|PSG_DAT|Write Data									 |
		   |  |	   | PSG Register 14 - Port A		    %RICDPBAS |
		   |  |	   | Reset IDE 0:no,1:reset (slow down)---+||||||| |F
		   |  |	   | Internal Speaker 0:on,1:off-----------+|||||| |F
		   |  |	   | Centronics Strobe----------------------+||||| |
		   |  |	   | Reset DSP 0:no,1:reset------------------+|||| |F
		   |  |	   | Printer Select In------------------------+||| |
		   |  |	   | Drive B select 0:on,1:off-----------------+|| |
		   |  |	   | Drive A select 0:on,1:off------------------+| |
		   |  |	   | Side select 0:side1,1:side0-----------------+ |
		   |  |	   | PSG Register 15 - Port B		    %xxxxxxxx |
		   |  |	   | Centronics Data Port-----------------++++++++ |

With clever use of interrupts (maxymiser tracker!), very decent music is possible. Basically you select the YM register with the upper register.
And then you write data to this YM register with the second ST address.

Write $01 to YM register $05 would mean

move.b #$05,$ffff8800 ; select Ym register $05
move.b #$01,$ffff8802 ; write $01 to this

Websurfing on lowspec Atari computers can also be improved by visiting the mobile versions of webpages instead of the full ones.
I also second that the recommended minimum configuration is 4MB RAM and a harddisk.

Lethal Xcess and Wings Of Death need to be mentioned, ace shooters and technically the best on the ST.