Let's see, I've been avoiding missle boat mechs for now because I know missle boats are probably what I will want to be playing..... Excluding the Catapult, which was my first. I knew it'd be my favorite because of it's design and I guessed that I'd love it's gameplay style the most.
Here's what I look for in an LRM boat:
#1 Enough weight to deal meaningful damage with LRMs (efficiency of LRMs, and a decent amount of LRMs)
#2 Enough Speed + Manueverability to stay with your team, and to distract the enemy long enough for them to be killed by your team if they focus you, since people often focus LRM boats.
#2.5 Jumpjets - Torso weapon focused mechs will hate you, and helps you re-locate/escape better.
#3 Ability to use Tag Laser
#3.5 Some extra energy hardpoints for more medium lasers for self defense to discourage attackers. Don't be that guy that can't finish off something with 1 hit left while they whittle your health down from full health because you had no backup weapons. I suggest at least 3 medium lasers in general, enough to deal some respectable damage and finish off people, while not taking up much room/tonnage. Ballistics are too heavy if you're trying to be an LRM boat (and Machineguns aren't reliable self defense).
#4 The correct size for tubes for what you are trying to do. Streaming is fine if there's no AMS (which is common cause people care about dealing damage too much), but if you want a massive load, you want big tubes.
However, out of the Catapult, Thunderbolt, Cataphract, and Atlas that I have mastered, here's what I can say about them.
Atlas D-DC is the only Atlas that I would even think of doing LRMs on. However, even then I'd never go full on LRMs with nothing else, don't go full on boat. Since it has ECM, allies will flock to you, making it so you never really have to worry about being protected from lights (unless you're being such a tard that no one will follow you). Back when it was 8v8, I used AC20, PPC, tag, and 3 LRM 10s (probably a good idea to downgrade 2 of them to LRM 5 cause of tubes though), and it was practicly a steamroll everytime since people would deathball around me, and anything that was any threat I would just use the PPC + AC20 to finish them off, most of the time I never saw the enemy but got more damage than most players all while constantly running at the enemy. Probably wouldn't work so well with 12v12 though.
ECM is really the only thing going for an Atlas LRMer, but at that point I still suggest some other build if you actually want to be effective.
Cataphract2x, LRM boating? You're kidding, you're kidding right? Moving along....
Catapults in General:
Personally these my favorite LRM boaters I've used so far, and my favorite mechs. Slap in an XL 315, add some jumpjets and you can get away from anything that isn't an upgraded light. You're also just the right speed to get people to chase after you, and it's easy to stick with the team. Also, remember to open those doors, so it doesn't feel like you are lagging when you shoot.
However, Catapult was the first mech I mastered, so I may have to go back to it to make some better builds when I get some more mechbay slots, even if the mechs already felt perfect as they were.
A1 - Personally I have not tried the 6 LRM 5 build yet (Played it when Splatcats were popular, tempted to do all SRMs, or quad srms with some LRMs during that time), but I've seen people use it to great effect. Supposedly it doesn't need the Tag Laser to have effective LRMs (which wouldn't surprise me but I haven't tested it). Also, the psychological effect of being hit with a stream of these is pretty massive and is one thing the A1 excels at.
C4 - This was my go-to LRM boat for a while since it was just a bunch of missles, and could have a tag laser (which I love to use cause Lights love to think they can outrun LRMs). This was before I wanted to have self defense on my LRM boats, and just wanted the hardest hitting loadout.
C1 - Originally I prefered the C4 over this, but I may revisit this Since the Thunderbolt 9SE and some other mechs made me fall in love with slapping medium lasers on LRM boats,
Thunderbolt 9SE - This is the wannabe Catapult if you try to LRM boat this. Though, considering all your weapons are in one torso, and your side torsos are easy to hit, just use a catapult. However, when I was using this, people would absolutely charge into my team trying to kill me everytime, while they get massacred from focusing on me too much (and I'd usually come out of it without much damage anyways). However one nice thing about the Thunderbolt is that it's arms can be used as shields very easily, so if you are good enough with torso twisting, the side torso problem isn't too big of a deal.
Everything else is just me taking a look at smurfy, going off of my experience from fighting them and guessing how effective they'd be.
Haven't done a true Light, Medium, or Assault LRM boat yet, but here's my opinion on them for now:
Lights: I will only slap in a single LRM 5, just for psychological effect and to lure someone towards you since it's damage is too small to really phase anyone. You'd be better off using Lasers for damage.
Mediums: I usually see people slapping on multiple LRM 5s to any of these, but nothing bigger than that. Though, I will say Kintaro LRM boats are the most annoying Medium LRM boats to fight against.
Assaults: Considering how positioning is so important with LRM boating, LRM boating on an Assault seems like the most luck based build in the entire game. You'll get massive damage when you are able to hit someone though, that's for sure, but you have soo much weight you have no excuse not to bring backup weapons.
Out of the mechs I see right now, here are the ones I expect to do well with LRMs.
Lights: N/A, you just don't have enough weight for the ammo required to really boat.
Mediums: Trebuchet 7m, Griffins (depending on what you want), Kintaro (except 20, 5 LRM 5s are a pain for your enemy), Shadowhawk (depending on what you want), Wolverine 7k
Note: Mediums have a lot of variety when it comes to what you want to do with an LRM boat. Also, Centurions, you are zombie masters, get some SRMs and show your zombie powers. I suppose you could LRM boat with them though.
Heavies: Catapult (No Surprise), Thunderbolt 9SE (and maybe 5S), Maybe Quichdraw 4h, Maybe Orion
Note: Jager A I'm afraid to put here because I worry for it's self defense, since Ballstics are probably too heavy to keep being LRM heavy, and there's only 2 energy slots.
Assaults: Battlemaster 1S, Highlander (self defense problems), Awesome 8R (self defense problems),
Note: Stalker is a strange one because it has such tiny tubes on it's missle hardpoints. It used to be the most popular missle boat (and you still see it a lot). If you don't mind streaming a little, it has plenty of backup weapon slots.
I will probably always like Catapult the most for LRM boating, I find it to be the sexiest Inner Sphere design and the perfect blend of manueverability and firepower for hard hitting LRMing. It'll be replaced by the Mad Dog whenever it gets in, too bad Mad Dog can't get Jumpjets though, I love being in the air.
Also, if the LRMs are fired like they were in MW2, all the more reason for me to want to use the Mad Dog, however I forsee a slight rise in AMS if that happens.
*continues the search through Sarna in case his dream mech is still hidden somewhere*