The key factors with Walkers, in my opinion, are price and rate of fire. They are a fairly fragile unit, so you don’t want to be putting a ton of points in them. Also, they only have a BS of 3, so you only have a 50% chance of hitting with any shot. With Shuriken Cannons and Scatter Lasers you get 3 or 4 shots respectively, and this goes a long way for making up for the low accuracy. They are also the two cheapest options. Once you start paying more points for different guns, you are losing out on the number of shots fired, and really lowering the effectiveness of the squad.
In my opinion and experience, the dual scatter laser walker is by far the best. You get a ton of shots, that can easily take out hordes and wear down marines as well. Plus being strength 6, you have a good chance of taking down big bugs and tanks as well.
The next best is shuriken cannon walkers. These are so cheap it’s almost a steal. They make great outflankers because of the range. They lack the ability to stay out of range of the bolters and such, but if you’re outflanking then that doesn’t matter as much.
Third would probably be EML walkers. I’ve had less that stellar luck with these, but against large hordes, the 6 blast markers should be able to do some damage. The krak missiles can take out light and medium tanks if needed. The range is the biggest advantage, but I don’t think it’s worth it.Bringing up the rear are Star Cannons and Bright Lances. These fall into the category of just being too expensive for what they do. One shot with the bright lance isn’t reliable enough to take out heavy tanks, and the fact that a single lance costs more than the walker itself makes them less than optimal. Without a Farseer with Guide, I would not rely on these to do their job, and for the cost there are much better options in the Eldar army. Star Cannons have a similar problem. They look good against Marines and Terminators, but they low rate of fire and price makes them less than optimal. Plus, this is only good against units out of cover. Once cover saves get involved, then they lose any advantage they had. So unless you play against marines on a bare board, I would not recommend Star Cannons.
As far as mixing weapons go, the best combination is probably Scatter Laser + Star Cannon. They have the same range, and the Star Cannon adds some AP to the mix for big bugs or terminators. A problem, though, lies with the 5th edition wound allocation rules. If you’re firing with both weapons, the opponent might be able to stack the wounds so that only one model gets removed from the armor piercing hits.
Scatter Laser + Eldar Missile Launcher (EML) is a decent choice, because it does provide a bit more medium tank killing power, and you can still take on hordes well enough. You do lose out on the extra shots, though.
War Walkers are great units with an awesome model, but they do succumb to a few issues that make certain weapon combinations less than ideal.