Depending on your budget the new $5 $10 & $25 gold buffalo's are beautiful new coins and I think the new small denominations will do well with value over the years. We wont know mintages for a while yet, but I bought several myself and i love them. lean more toward the $10s and $25s.
Small denom. libertys and even the older indian eagles are great old coins. But again i stick with smaller denominations. Unless its somthing rare, most 1 oz coins go about 10-20% over melt. Smaller denoms of these older coins ive seen higher percentages per coin plus you get more coins for the same investment as one 1 oz coin.
Obviously coins with lower mintages are usually best, this goes for old and new.
Grading is nice for rare coins, low mintages, & proofs/mintstate's. But when it comes to the newer coins you can almost always find graded coins for about the same as non graded.
Last but not least, ebay - it can be a great place to buy, but be ware. If its seems way too good to be true, it usually is, but otherwise ive bought coins for over 10% below melt, gold and silver.
The newer $5 gold eagles can easily be acquired below melt and some people are unknowingly dumping off ones with fairly lower mintages. Great buys when you can buy 10 of these for less than melt as opposed to a 1oz coin at 5-10% over.
Now granted gold is down right now, these percentages have shrunk a little im sure, but still a good time to buy if you ask me. At just over $800 an oz, i dont see it getting much lower ever again.