Put a cheap 35 ton Hunter vehicle on the table with an LRM-20, and the Valkyrie is effectively obsoleted in most situations.
Except when it needs to go up the mountain, be kicked out of a dropship, cross a river/lake, get through heavy woods in a tactical timeframe, or operate in a hostile environment.
The OP has also not asked for the best- but rather cheap/abundant workhorse. We see plenty of Valkyries in mercs IIRC, so because they were one of the most produced mechs in the 3SW they should be abundant. They work pretty well as long range support for the scouting bug mechs.
IMO the Valkyrie use comes down to how heavy or light you want your force to be . . . to the OP, you said you like/want the Marauder- jump on that . . . and to get back to your OP-
Marauder- solid in the early eras, the 3 series has a few options you can convert to (or start with) depending on your supply situation. The -3D, -3L and -3M (even -4X) all offer options that you can take your Marauder even before 3050. After the tech starts to become available again int he 3040s leading into the Clan Invasion of the early 3050s, you start getting a LOT more options- like 15 more in the following eras
so far. I THINK it might be the mech with the most variants if you use the command/DC/named options with numbers in the low 20s? This will be available in one of the new KS boxes.
Role: Command & Direct Fire Support (for most)
Griffin- Very good ranged fire support in most variants that can also drop behind cover for indirect fire support. During the 3rd Succession War (early era) you only get 2 variants really, but after that it takes off for a total of 13 with a few unique versions. Produced all over and a long established design you should be able to maintain one even in the tech poor eras unless you have a bad run of luck. The Griffin is one of the classic trio, pretty mobile since it can do a full jump and has hands. You can also get this mini right now in the Starter Box if you do not have it already.
Role: Skirmisher & Fire Support
Locust- You said you like the chicken walkers, so why not? As mentioned its a speedy recon machine though its not the fastest, it can get pretty close to that claim in about any era. It has a lot of variants, but what they all have in common is speed & a mid ranged weapon- usually a ML. As mentioned, its not a stand & slug it out design but can be pretty useful as long as that is not the only type battle you are involved in- even then it has uses for a player who thinks. Later eras? It gets a bit better and keeps up with the speed . . . I LOVE the Locust 5M but you have to becareful with them since speed is not as effective a defense as it is in early periods. This mech is found in the Game of Armored Combat box as one of 8 minis offered.
Role: Recon
Last mech for lots of variants & availability in BTU/mini should be a Shadow Hawk, Wolverine or Thunderbolt- just sort of depends on what you want to do and how challenged you want to be going into fights. All three are available in currently released boxes though the Classic Shadow Hawk has been released by Iron Wind Metals for a little while now along with a variant.
If you want to know what mechs are abundant and have variants through the timeline you can look to the mechs Catalyst put in the already released boxes (9 designs- Griffin, Wolverine x2, Locust, Commando, Shadow Hawk, Thunderbolt, Catapult, Awesome, Battlemaster) or in the coming KS boxes. The KS boxes have a few exceptions among the IS mechs (Nightstar comes to mind) but its a pretty solid rule.