If you've got money to burn, Steel Battalion.
Otherwise Battlefield 2142, G-Police, Gatling Gears?

Future Cop LAPD
This game started out as a sequel to the Strike games (Desert-, Jungle-, Nuclear Strike, etc.) and feels and plays like one. You can transform in a car/tank. I think this game is a lot of fun and has some very unique multiplayer modes I've been told.

Shogo
This is a first person mech game. A lot of people seem to hate this game for some reason, personally I love it and being in and out of mech gives a good sense of scale. Also the weapons you can use as a mech are really powerful.

Heavy Gear 1 & 2
These are simulatosr like Mechwarrior. For some reason I liked these games even more than the Mechwarrior games themselves, the mechs..., sorry gears are smaller than Mechwarrior mechs but you can drop and pickup weapons and such. Also I never cared much for the Dune-like backstory of Mechwarrior and Heavy Gear has more of a Wing Commander story.

Slave Zero
Already mentioned a few times. I only ever played the PC version, which according to LGR is far superior than the Dreamcast version, and I like this game a lot and never had any problems running it. It's purely an actiongame though.

Brigador
Fairly new and I think it's still in early access. Looks like Syndicate with mechs with 80's synth music in the background blowing stuff up. Maybe they should license this game to Kung Fury.

A bit off topic, but here is Steel Battalion's epic controller: https://upload.wikimedia.org/wikipedia/commons/e/e7/Steel_Battalion_controllers.jpg


Also, Titanfall again. Just play it.

Can't have many players anymore.
Isn't it dead already?
Might be doing better than Evolve is at the very least.

Unfortunently its dying, yes (PC version at least). There are usually less than 1000 players worldwide and most of them are centered in one or two modes. I got it few months ago from Amazon for 5$, I wouldn't buy it if it was higher (I'm more of a single player guy anyway) but I still play it sometimes and have a blast every single time.

You can play it for 48 hours for free on Origin, try it.

There's always Metal Fatigue! That was an interesting mech game in an RTS format.

I fear I wouldn't be much help, rarely play online games aside from co-op games and CS GO
I can't even remember the last time I used Origin. I even use Uplay more frequently.

Well, it's always available, just try it when you like to.

My favorite is Ultimate Knight Windom XP. It is a classic style arena fighting game similar to Virtual-On. You play as a giant mech (considering to the official site it is more than 30 m) for the pure purpose of mass destruction. There is 24 different mechs but most of them must be unlocked before using. Every mech has a primary and secondary weapon, as well as three sub weapon systems. The melee attacks can be chained or canceled like in the fighting games.

It is an indie game based on the classic Japanese mech world. It combines elements from Zone of The Enders, Evangelion, Gundam, Massin Eiyuuden Watoru, and so on.

The game also has online multiplayer for up to 10 players at the same time. Of course it has its own server creator tools, and use simple direct IP as connecting method. You can even make pilots from voice clips you have on your computer, and these pilots can be used online. :)

A video if you're interested:
Survival Gameplay

I'll second this. These games are pretty good, though I never got the opportunity to play MechWarrior so don't have much in comparison. Starsiege is the third in this series and a little more modern if you can find a copy on Amazon or eBay cheap. Also a very good addition.

Ultrabots is a little known DOS game as well, a little slower pace than Earthsiege but still worthy of some time if you can find it.

Shogo
G-Nome

I haven't played G-Nome in nearly 20 years...I need to replay it at some point.

I totally agree. That was the best mech simulator game of the 90s. You get to fight giant mechs on foot too.

Back when it came out in the 90's, it was a total gem. Too bad it did not get the praise and recognition it deserves.

front mission evolved

its an action spin off from a tactical rpg series only on the playstations
its okay
you get to stomp around i huge mechs
its not great but not complete garbage either

Front Mission's only problem is that you're stomping around in a wanzer. When mispronounced, this single word makes almost any scene in Front Mission 3 at least into utterly hilarious. "I'll give them a taste of my Wanzer!" "Business end of my Wanzer!"

This is not really to topic, but Super Robot Taisen is a really good franchise with some really cool giant robots.

Back more on topic, which Armored Core would be a good place to start? Does the first one hold up well or would a later game be more suitable?

Hawken on steam.