There is very little to no customisation nowadays (bar changing the colour of a gun) all guns now can be picked up out of the box and shot with no issues or no improvements to be made.
As far as what's top in the market its anything Planet Eclipse (LV1, Geo 3.5/3.1) followed by Dye DM series (14/15) and DLX Luxe. There are a few RSX shockers kicking about and some guys still rock older guns (pre 2010-I know that's not really old but in terms of tournament paintball guns its ancient)