Have a grizzly 1754, came with this 2007 15hp and a beat to heck prop. Wondering what the best prop might be to start with? Plan to upgrade motor soon. 

At that size of motor I would just get what it came with and call it a day.  Changing the pitch or number of blades with something that low of horsepower isn't going to gain you anything and will most likely hurt the performance of the engine.

It has an unmarked 4 blade prop currently. I'd like to put a standard 3 blade, just not sure what pitch would be ideal. 

You can get a 7p 4-blade or 8/9/10p 3-blade for the 15.

 

https://marine.honda.com/parts/accessories/categories/propellers

10 pitch should work well for you. We have several 10hp limit lakes around me. We all put 15 carbs on the old two stroke 9.9's to make 'em 15's, or get the 20hp computers for our 9.9 four strokes. Just about everyone with heavier boats like yours runs a 10P. Lighter boats do better with an 11P.

Try 

propwizard.com

propellerdepot.com 

both have finding the right prop links. You put in motor type, make, model, hull type, weight …etc, etc  It will get ya close. A tachometer is nice when fine turning. Have you taken the prop off yet? Most props have markings some where. Sometimes it’s around the spline area, or under the washer, even if it’s only a part number.