None is necessarily better or best, because they are somewhat different.
Miniatures Handbook is very miniatures-focused; the mass combat rules are about using LOTS of miniatures (or tokens). Being a hybrid of D&D and the minis skirmish game, it does not do much to discuss realms, logistics, or any other way of handling the battles.
Cry Havoc is more abstract and tactically oriented; it offers an abstract system for resolving war "offstage" as well as rules for the PCs being in the middle of the battles. It is very much about adding War as another element of a "standard" D&D campaign. Logistics, and realm management, are left "offstage" like the abstract war resolution.
Fields of Blood is about adding a dimension of realm rulership to the game. Its mass combat system is pretty comprehensive, but much of the book is about the "bigger picture" (political arena) in which war takes place. My main complaint about this book is that it is (sometimes) hard for an existing campaign to re-draw its map to the scale used in Fields of Blood.
Testament's mass combat system is more about heroics. The PCs are mythic-scaled figures, capable of feats that put them on a par with a unit. The feel is very different from Cry Havoc or Fields of Blood, and that is intentional.
In each of those cases, the feel is a bit different because the goals are a bit different, and the rules are different to reflect that.
Also:
Empire (from AEG) is similar to Fields of Blood, adding both mass combat and political maneuvering to the game.
Book of Strongholds and Dynasties (Mongoose) contains their OMCS II (Open Mass Combat System revised) rules. It is, however, a very small part of the book and probably not worth the money unless you also want the construction and domain rulership rules.
Finally, Trojan War (Green Ronin) has a 3.5 update to the mass combat system from Testament. Like the original, it remains very much a "mythic" combat system.
A lot depends on how you want war to figure into the campaign.
- If it is going to be the "backdrop" against which adventures take place, but the adventurers are not going to be in the middle of a lot of battles, then Cry Havoc may be just what you want.
- If it is going to be a campaign where the PCs lead a lot of forces and are expected to show incredible prowess on the field (ex: one blow takes out 20 enemy soldiers), Testament or Trojan War may have what you want.
- If the war is part of the campaign world's development, and you want to play up the political angles, Empire or Fields of Blood is probably more appropriate.
You may still find some use from the Miniatures Handbook, even if you do not want the mass combat system. It introduces some spells for affecting formations of troops, and the Marshal class is a reasonable approximation of a commander-type character (the scenes in The Two Towers movie, of Aragorn with the elf troops, are how I see the Marshal playing out).
