For TBs and similar "hot" horses I like a grain-free diet. You can accomplish this with a vitamin supplement and hay pellets or beet pulp, or a concentrated "ration balancer" feed/supplement. These are fed at the rate of 1-2 lbs a day for full nutrition.
I use ADM's StaySTRONG Mineral Pellets. You can order them online if you don't have a local ADM dealer. One bag will last about a month per-horse, and it's only about $15 a bag, so the price is very affordable.
Other companies that make a ration balancer or "supplement" feed include Progressive, Triple Crown, Buckeye, etc. I don't care for Purina's ration balancer. It has too many fillers and isn't a fixed formula (never know what's going to be in it...).
For just a simple vitamin supplement, you could go with GrandVite, Select II, or SmartPak's SmartVite Maintenence Grass.
If you want a traditional "feed", look for one that says "Lite" or "Low Starch". You need to feed to the bag's recommendation for full nutrition. Most Lite feeds recommend 3-4 lbs a day. Look at the feed tag ingredients. Stay away from feeds that contain corn, molasses, any wheat products, and barley. All of these are high in sugar and/or starch, which can make a hardkeeper lose weight!
My TBxArab gelding used to be a hard keeper. We fed him buckets of food and supplements just to keep his weight steady, without much real "gain." Now he holds his weight beautifully on free choice hay (no real grazing), 1 lb of ADM's StaySTRONG pellets, 1/2 of a 3qt feed scoop of Timothy hay pellets (about 1.5 lbs), and a couple squirts of Uckele's Cocosoya oil (for added Omega 3's, since we don't have any grazing).
My hard working horses get the same, but instead of Timothy pellets they get 3 lbs of Alfalfa pellets. They are "cooler" horses though and the added Alfalfa doesn't hype them up like it does my TBxArab.