Going to speak in general terms since this is a rather broad subject.
Well, if a slow attrition match is a concern, try to stay away from everyone taking assaults
(and depending on game size even heavies), thou an odd 'Big' 'Mech on each side can
sometimes speed the match up, as they can have enough punch to knock a lighter 'Mech quickly.
Most of the 'Unseen' 'Mechs (the ones based off of Macross, Dougram, ans Crusher Joe art)
tend to be favorites not just because of their looks but the fact their game stats lend them
to be effective designs.
There are a few other designs out there that are general favorites. The Hunchback can
be a good introduction 'Mech. It has one big gun, you run at the enemy and you try to shoot
them with it. The Vindicator, Enforcer, and Centurion are all good starters as
well, being decent all-around choices.
Another idea would be look at whatever factions the players think are interesting and grab some
'Mechs that they use as a departure point to start narrowing down the list.
If nothing else, grab some mediums and have at it, the players (and well as yourself) will
come to choose some favorites in the course of a couple games.
As for balance, 3025 'Mechs can be reasonably balanced by both sides keeping to the same
total tonnage (and the same number of units) , but if the material you have has Battle Value (BV)
info for the designs on them, that might be a safer bet for balance (and a habit to get into for
matches where opposing sides have different tech).