The most useful XL engines to have are:
- XL255: It's the first XL with 10 built-in heatsinks inside (and weighs the same tonnage as XL255). Some lights using heavy weaponry (Panthers, Ravens), mediums (HBK-4J, Cicada, some Cents etc) and some slow heavy builds (4xAC5s and such) utilize this engine.
- XL280: It sits mid-way between the 255 and 300 and is used mostly on 55 tonners with heavy weaponry (Griffins, SHDs, Kintaros, Wolverines) and some lights. Not a priority, but it's nice to have.
- XL295: Max engine for some lights (Firestarters, Ravens) and mediums (Blackjack, VND-1AA). If you want to run lights you'll need this a lot.
- XL300: The holy grail of the XLs. The XL300 sees so much use between all the classes and sits at such a sweet spot that you can't have enough of this engine. Lights (Jenners), mediums (pretty much all 50 and 55 tonners achieve optimal mobility/tonnage with this), heavies (QKDs, DRGs, JMs, CPLs . . . ) and even some assaults (well, mostly for joke builds) use these engines a lot. Buy as many as you can.
- XL325: The first XL with three heatsinks in it. While mediums can rarely use it (Trebs usually do), heavies and assaults use this engine a lot. It's especially useful when you're building an energy boat as the extra slot in the engine lets you cram more DHS in. It's also the max engine of the Mauler and the Highlander, two very popular chassises.
The rest is optional. But I'll list them anyway.
- XL360: This is the max engine for many 'Mechs and you WILL need this a lot. It's the maximum for many mediums (55 tonners), heavies (GHR, DRG, GHR, BLK-KNT, Orion) and is also used frequently in assault builds, because 360 is where assaults start getting responsive.
- XL190: Locusts use this. Some commando builds too.
- XL235: Heavy usage in Blackjacks and Vindicators. Some min-max medium and heavy builds(Jagermechs) will need this too.
- XL240: A useful engine to have before 250. Commandos use it to reach 170 KPH as well.
- XL340: Battlemasters and Banshees use this engine a lot.
- XL350: Mostly used in Victors, first engine with 4 DHS in it.
- XL370-375: A good "gap filler" engine that is rarely needed.
- XL380-385: Fast assaults use these a lot.
- XL400: Very frequent use in Banshees and Battlemasters. The only engine with 6 DHS in it, so big energy boats will need this.
Also read Terciel1976's corrections as well. I made some boo boos and he also has a different perspective on things.