I think 3025 introtech is the ideal place to start new players at.
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I think the Beginner League should always be 3025, and that there should always be a 3025 game going for new players to drop into.
I completely agree.
A lot of people will advocate for Clan Tech &/or flat out Repulic Era because that is the "fiction" era, but to me, what makes a good player is mastering movement, balancing heat, knowing how to keep your TMM higher than you AMM, using terrain, etc etc.
If there was one out there, I'd use a "Fire Javelin w/o JJs" as my ideal trainer mech.
There isn't so instead, I'd substitute Mongoose-67 or Wolfhound-1 as canon mechs that are close to that.
I don't want them getting addicted to JJ overuse, I don't want them fiddling w/ 4 different weapon bands, I don't want them in slow 4/6/0 Mediums as much as those mechs are suggested for "trainer" games.
I want them in something that is "fast-ISH" and has simple weapons so they are learning how to position on the board.
Because watching someone make a bad movement & leave themselves at a +1 TMM (or 0) when they could have been +3 & then get pasted by an enemy lance both hurts your team chances at winning but also isn't helping the player learn really.
Knowing when to go around, go over, hide in, or stay behind various terrain features, and for that matter, when NOT TO, is one of the things that can separate a one player from another in terms of skill.
If you can master playing (killing with) 3025-LIGHT MECHS then Heavy/Assaults &/or Clan Tech is all "Easy Mode"