MW2 has the best music in the franchise, I still tend to listen to that on the CD. As for the game itself, MW2 was the first one I played and my first contact to BT universe, so I might be biased to say it's great for its age and has the best single-player experience in the series. You get lots of fluff concerning the Clans and the history of BTU, the atmosphere is wonderful and immerses you into the story, but then seeing buildings and 'Mechs suddenly appear out of nowhere breaks the immersion - the engine is not powerful enough to model weather effects, so it might be completely clear skies but if mission briefing says visibility 300 meters, then 300 meters it is. Literally, with no "fade-in" whatsoever. Broken Munchkinlab aside, the missions simply compensate for the limited AI by giving the computer opponents (sometimes ludicrous) numerical superiority, especially evident in the Trials of Position: at the last one you pilot a Firemoth against three waves of several Dire Wolves (first one, then two, then three, or something like that). Never got to experience the multiplayer, though.
But in the end, MW2 captured the spirit of BT in a way most of its sequels didn't, and that's what matters the most. MW2Mercs is almost as good in this regard, just from the opposite perspective.
MW1, despite its sandboxiness, really shows its age in a bad way. Of course, the graphics will blind you and the PC beeper effects will deafen you, unless you started your gaming in the 8-bit era, but that's kinda expected and is not the real issue. It might have been groundbreaking in the 1980's, but even by early 1990's standards its small number of units at play (maximum one lance vs. one lance IIRC) feels too limited.
MW3, nice FPS. Oh it was supposed to be a 'Mech simulator? Not. Storywise, not as epic but still felt a part of the larger story.
MW4, felt too clinical in the single-player mode. Problem is, while in MW2/Mercs and MW3 the fate of the entire galaxy was at stake, now it's just your title and meager fiefdom. Omni vs. "type" slots, slot sizes, and limited number of slots was a nice idea to prevent total munchkinism in the 'Mechlab, though. No multiplayer experience, skipped directly to Mercs.
MW4Mercs, Solaris VII was a nice addition as was the branching storyline, but still, in the 2000's one would have expected more sandboxing MW1 style. And the story still felt a bit disappointing in single player, only in the very end you really feel connected to the Big Picture. Multiplayer with MekTek paks was lulzy, by the time I had beefy enough PC there were too many Stone Rhinos with two Sniper cannons (in the long-range maps) or Dire Wolves with something like 3xCLBX20+3xCLBX10 (short-range maps) around.
MWLL has the best multiplayer hands down.