Simply based on sales, where I work, the 500 Mossberg and 870 Rem Express are still running neck in neck as THE starter shotgun.
As noted, the Maverick is less expensive, but not by enough that I would choose it to save 100 bucks - you do get what you pay for even in lower end shotguns.
I would stay away from the 887 Remington Nitro Mag's - they look mean but after assembling a half dozen of them the "theory" that a synthetic stock is better than wood because it won't warp etc in non-sense - these things are so "twisted" that it takes "great force" to get the barrel/forend/receiver together - at least on first assembly.
On the other hand, an 870 Wingmaster (costing at least twice as much as the 870 Express in most of it's various models) shows definite signs of "refinement" - the fit is awesome, so in my opinion, it's money well spent.
There is also the Mossberg 535's out there in a few different offerings - they are basically the 500 model with the super mag chamber (3 1/2") - so if you just have to have a "super mag" there is a Mossberg version to compete with the 870 supermags.
Likewise, for a first shotgun I would stay away from the Semi's. Cut your teeth on a "reasonably priced" pump and if you find you have a need for a semi, spend some bucks on a good one - you don't want to cheap out on a semi. A cheap pump will be functional, a cheap semi will be a potential nightmare.
Don't overlook some of the less common brands. Weatherby makes a pump (PA-08) in synthetic and hi-gloss (upland) stocks. The wood version looks every bit as nice as a Wingmaster, the fit/finish is excellent and it costs about the same as a 870 Express - so pretty, but economical.
There is also the Nova from Benelli but the price is starting to get up there on their "entry level" gun.
Of course Winchester and Browning also have shotguns. The relatively new SXP from Winchester is meant to compete with the 500/870's - never sold one and don't know anyone who owns one so can't say - but pricewise, they are in the running.
For just a few more bucks you could go with a BPS from Browning. Good fit/finish and have never heard significant complaints about the function.
I own a Rem 870 Express and Wby PA-08 Upland.
If I was buying a new utility shotgun today I would choose either another Weatherby or the BPS from Browning. After assembling dozens each year, those two just go together "properly" right out of the box which indicates they are just a little further up the quality control ladder than any of the other entry level shotguns.
(edit - neglected to mention)
If you have 600 bucks in the piggy bank you may want to take a look at the Mossberg 535 3 barrel combo gun.
It has the 3 1/2" chamber and three barrels - a 28" Vented rib, a fully rifled deer barrel with cantilever (so you can mount a scope) and a Turkey barrel, complete with extended choke tube and fiber optic sights (can't remember the length of the deer and turkey barrels - somewhere in the 22 to 24").
So for (about) 600 bucks you get a shotgun that will "do it all" and it's dressed in a pretty cool looking camo pattern to boot.