Probably Skirmishers and something that isn't a pushover in melee. Even if the Ratha goes melee mode, its armor type doesn't change so Skirmishers are still effective.
What about Pikes/Skirms? Onager?

spear+skirmishers make short work of them as do a lot of unique units.

Micro with skirms

I think skirm + spear is the main counter. Note that ratha has less damage bonus vs spear than most other ranged units, and like KingKickAss said, it still takes damage bonus from skirms even while in melee mode. It's also much more expensive than crossbowman without having any more firepower, so it really doesn't kill spears too quickly IMO. Also monks are effective in small numbers for hte same reason

I think that overall rathas are no match against some types of elephants (especially ballista elephants which is a unique unit but even against battle elephants and war elephants you could try to hit and run with ranged ratha but the problem is that these elephants despite being slow have a lot of hp), except for siege elephant and maybe elephant archers (because despite them having a lot of hp they are slow, much weaker overall than ballista elephants, and could be vulnerable to melee ratha attacks). They are also weak against camels (because the tech tree labels them as such but you might need to mix in other units with camels against rathas because they are also ranged and camels have a weakness to ranged but this could perhaps be less problematic if the rathas are being played by AI instead of a human player), paladins (but not certain about cavaliers because even with full upgrades cavaliers at best I think are only slightly stronger than elite rathas) and savars (similar to paladin but cheaper to upgrade and slightly stronger) skirmishers, pikemen (plus upgrade), and various other unique units as well.

To speak to training time, majority of castle units train fast, some very fast. Simply because your limited on castles. Depending on the map you'll have between 3 and 5. Sometimes 6. While you can easily have 10 archery ranges pumping out skirmishers.

Pretty sure the Knight line just outmuscles them.

Originally posted by Crossil:

Pretty sure the Knight line just outmuscles them.

Especially paladins and savars. I think cavaliers with full upgrades might be a little stronger than melee rathas, but not by much. Even though melee rathas have lower hp you also need to take into an account they have 3 armor vs 2 armor and have faster attack speed than cavaliers as well from castle tech in addition to the elite upgrade having 12 damage which identical to damage from cavaliers.

They're still cavalry archers in melee mode, Skirms always hurt them

Originally posted by Devang:

They're still cavalry archers in melee mode, Skirms always hurt them

True, but Rathas are potentially weak against other units as well and not just skirmishers. Camels and skirmishers against rathas for example is a good combo. Even pikemen and halberdiers are potentially good against rathas when mixed with other units, because Bengalis do not get Parthian tactics anymore. Rathas are only good against militia line infantry, pikemen (but it is harder to deal with them when mixed with other units), siege, light cavalry, elephant archers (because they are weak to melee rathas) and other archers, some unique units, and from ranged ratha also good against monks.

Walls.

No but seriously Skirms with any meatshield will resource effectively dunk them. Spears, even light cavalry smashing into them if you need something cheap while Skirms work them over.
Ratha like a brick of Elephants in front of them but it's an expensive combo vulnerable to Pikes + Skirms (yes, even with reduced bonus damage for the elephants) and Scorpions/Onagers. No shame being cheap on the Bengali side and smashing Pikes into stuff before Rathas roll in.

Originally posted by Heimdall313:

Walls.

No but seriously Skirms with any meatshield will resource effectively dunk them. Spears, even light cavalry smashing into them if you need something cheap while Skirms work them over.
Ratha like a brick of Elephants in front of them but it's an expensive combo vulnerable to Pikes + Skirms (yes, even with reduced bonus damage for the elephants) and Scorpions/Onagers. No shame being cheap on the Bengali side and smashing Pikes into stuff before Rathas roll in.

Melee rathas are good against any siege contrary to ranged ratha which even include scorpions (although scorpions with ballistics are good against ranged) and mangonels. Scorpions and mangonels are the most vulnerable when they are exposed and out in the open.

Originally posted by James3157:

Originally posted by Heimdall313:

Walls.

No but seriously Skirms with any meatshield will resource effectively dunk them. Spears, even light cavalry smashing into them if you need something cheap while Skirms work them over.
Ratha like a brick of Elephants in front of them but it's an expensive combo vulnerable to Pikes + Skirms (yes, even with reduced bonus damage for the elephants) and Scorpions/Onagers. No shame being cheap on the Bengali side and smashing Pikes into stuff before Rathas roll in.

Melee rathas are good against any siege contrary to ranged ratha which even include scorpions (although scorpions with ballistics are good against ranged) and mangonels. Scorpions and mangonels are the most vulnerable when they are exposed and out in the open.

I don't think anybody throws siege out there without, at minimum, a dozen or so pikes in front of them

Originally posted by Heimdall313:

I don't think anybody throws siege out there without, at minimum, a dozen or so pikes in front of them

Then you switch then to ranged ratha to take out the pikemen and ignore the siege weapons at least temporarily to get those trash units out of the way; although, scorpions with ballistics could still be a potential problem for rathas when mixed with pikemen, battle elephants, and/or camels depending on how many scorpions are being used meaning that scorpions could potentially be more problematic for rathas than mangonels which I think are less accurate than scorpions with ballistics despite having more area damage than scorpions. Pikemen are only good against melee ratha, because ranged ratha can hit and run assuming it is a human playing with the ratha instead of AI due to the fact that the AI is kind of dumb when it comes to ranged units in comparison to a human player besides maybe with skirmishers. Also, the way rathas are specifically designed the AI probably sucks with them in comparison to a human player regardless of difficulty settings. Rathas in my opinion are an overall decent and versatile unique unit, but it depends on whether it is a human using them or AI.

Against few Ratha meat shield + Monks are awesome. You get a free expensive unit

Skirmisher are the best counter