I've dropped that information some while ago somewhere alse, but here's my personal observations of the various weight classes.
Light: Vulture, Harasser, Spotter. Extremely flexible in positioning and completing team objectives. Very vulnerable and usually stays out of frontal combat.
Medium: Harasser, Flanker. Mobile 'Mech with moderate firepower. Can quickly reposition itself and can quickly adapt to most combat scenarios. Very versatile.
Heavy: Striker, Sniper. A powerful cross between the mobility of a Medium and the firepower of an Assault. The primary offensive force of a team.
Assault: Brawler, Support. Massive amount of armor and heavy weaponry make Assaults the muscle of the team, but they are impaired by their sub-par mobility. Take the front line to keep the smaller 'Mechs alive.
Each 'Mech class also has a native vulnerability:
Lights hava a disadvantage to Mediums. Mediums are beaten by Heavies which are, in turn, vulnerable to Assaults. These can however by torn apart by Lights. Of course there are tactics and loadouts that will act outside the more common uses and might align them differently on the scale. Thus this caparison is not absolute, but handy to visualize the pros and cons of the various classes.
Lights and Heavies as well as Mediums and Assaults have advantages and disadvantages that set then on equal footing as polar opposites. Engagements between these are heavily influenced by skill and loadouts, even more than the list above.
But back to topic:
Given that each 'Mech class offers different playstyles, there is no absolute on which 'Mech you should start with, though in light of the pros and cons, heavies would be the best place to start as you can carry over a lot of the things you learned about positioning to Mediums and things you learned about rules of engagement and trigger discipline to Assaults. When I started playing, I bought an Assault and a Medium, which yieded roughly the same experience, though it feels more difficult to combine these two together when playing Heavy.
Among the Assaults, the Stalker is the most versatile and flexible when it comes to loadouts and allows for a lot of experimentation. The only exception is the 3H, which is clearaly designed to be a dedicated missile boat. If that is the class you want to start with, the Stalker in almost any variant (4N is still useless and 3H, again, is a dedicated missile boat) is an excellent 'Mech for that.
I personally hava a
5S designed for all range fire-support, a
Misery geared towards brawling and front line support and a yet to be completed
3H missile boat. My builds are somewhat oddball, but they work quite well for me.