Generally I prefer the Large Pulse to the ER Large. If I've got the weight available, it's the pulse every time. The ER Large isn't bad, but the pulse is better. I can't remember ever using the Heavy Large. To me the range and accuracy and heat problems aren't worth it for an extra 6 points of damage. If I really need that extra punch, I'll find 2 more tons and get the ER PPC. The Heavy Large might be worth it in certain rare situations, or maybe on certain designs that I don't really use, but it's too situational for me.
With mediums, I think the competition is a lot closer. ER Mediums are the best (somebody said they're just "average", but I disagree -- they are so good that they set the gold standard for efficiency now). Medium Pulses have the accuracy advantage at most ranges, and they're a little cooler, but they're twice the weight. That's a lot. I can have 8 ER Mediums in place of your 4 Medium Pulses. Or 5 ERs and 3 extra DHS. I think the ER is more efficient overall.
The Heavy Medium has a lot of potential. 10 points of damage in a 1 ton energy weapon is just extremely nice. At that point, it's worth it to get really close and just trade damage with the opponent. It's worth overheating as long as you're taking shots at good numbers. I'd happily risk going into shutdown rolls territory if I had enough Heavy Mediums. I still don't think it's quite as good as the ER Medium or the Medium Pulse, but it's nice. Whereas the larges are ranked 1... 2......... and a very distant 3, the mediums are 1a... 1b... 1c.
The small lasers are basically different weapon systems from one another. My ranking here is going to be a little weird. The Small Pulse Laser is the best, followed by the ER Small, and then the Heavy Small is a distant third. Technically if you just look at range and damage, the ER Small should count as the best, but it's directly competing against a much better weapon -- the Clan ER Medium Laser. The ER Small only shines when you've got a half ton left over and you don't know what to do with it. If you've got a full ton, it's almost always better to take a single ER Medium rather than 2 ER Smalls. It's a little less damage overall, but the range is so much better that you'll still do more damage over the course of the game.
The Small Pulse is one of the best anti-infantry weapons in the game. It's not so great for fighting Battlemechs, but the ability to cripple a platoon at easy to-hit numbers is very nice. It's specialized, but it really shines at that specialty. The ER Medium just can't compete at that one particular role, and it's not that rare a specialty either. That's why the Small Pulse is the best to me.
The Heavy Small is a gimmick. The ER Small is almost always a better choice. The only real advantage is that it's dirt cheap. There should probably be a battle armor or something that carries Heavy Smalls. That would be freakin' awesome. But for any unit that has a choice between the two, the ER Small is better.