I may start hunting this fall with a crossbow. What is the best level of accuracy that is possible with the top crossbows? I have never owned one before.

There's a post by a guy who just started shooting a PSE TAC 15i crossbow here and he's got pictures of inch and three fourth groups at 50 yards and just under three inches at 60 yards. Does that sound reasonable?

Yep, I worked on a string for someone in Italy that wanted to shrink his 3" groups at 100 meters out of his Excalibur Exocet 200. With some work it got down to 1.5". Once the arrow is matched to the bow groups like the TAC should be easily possible with a good bow. It is all about shot to shot consistency.

in perfect conditions, no wind, good rest, etc, etc, you can probable get 3 inch groups with an Excal at 50yds but i would never shoot at a whitetail deer at anymore than 25yds.

Most bows are capable of shooting much more accurately than the shooter. The trick is to tune the arrow to the bow,purchase quality equipment and developing an effective shooting process, i.e consistent sight picture, trigger release, bow grip.

That is amazing on the accuracy. The archery shop I bought my fathers Excalibur at obviously is not away on how to properly tune the crossbow and arrow setup. The best I have been able to get out of his crossbow is about 5" at 50 yards. That is on a calm day with little to no breeze. If the wind kicks up at all that is exagerated even further. How would you go about tuning this crossbow to increase accuracy? I thought about getting one for myself but I can't hunt with it and with the grouping I had no benefit accuracy wise.

Make sure all the bolts and screws are tight on the bow first. Then address the arrows.
Cropssbows perform best with an arrow that has high Forward of center (FOC) ratio, like 16 to 20%, 4 bladed, low profile broadheads (small) and smallish vanes, like 2" Blazers. Low FOC and big vanes and heads can cause the arrow to rotate wildly around it's rear axis and plane unpredictably.
I'm getting phenomenal accuracy-from 3 bows-using 405 gr arrows: Gold Tip Lazer II, 110 brass insert and Blazers (+18% FOC). I generally hunt with 100 Gr Slick Tricks or Spitfires. Consistently hitting (the nose) on my bag target with a deer applique at 30 yards with 5 arrows in no challenge if I do my part.
PS for the Cyclone I just up the head weight to 125 grs, and aluminum moon nocks for 440 grs (20% FOC).
http://www.lmariana.com/foc.htm

Accuracy intrigues me. As a former bench rest rifle shooter/reloader, xbow accuracy was just another hill to climb. I shoot Excals and my favorite of them is the 200# platform.

My best groups to date were 1-1.5" benched at 50 yds, 5 shots per group. This was w/ GT L II's, 2" Blazers rt hand helical [AZ-EZ] and 100 gr field points. Y-25. Same basic arrow as Cossack favors.

Once you screw a bhead on, this can change. He's done a ton more testing than I have, I can put a Slick in a tennis ball out to 35 yds which is my limit, and I'd prefer 35'.

The point is, you more or less tune the arrow to the xbow. Others have their favorite recipes that work as well if not better for them. I quit looking once I found what I needed. YMMV.

Well I got a Parker BB 175 and with red hot crossbow bolt I am getiing 2" groups at 40 yards... Havent shot it past 40 yards yet...

I would not be surprized if it still shot around 2-3" at 50 yards... Bow is one bad mo fo

Accuracy with a X-bow is determined by several factors, all which have been listed above. Matching the bow and the arrow is the first step in accuracy. The next is your scope. You can't expect superior results with inferior equipment. This all being said, I can shoot a 1" or less group indoors at 40 yards...I personally never take a shot that far in the woods and keep my shots to 30 yards or under. 30 yard outdoor shot groups are 1-1.5" groups.

A bipod and a great bow and you can get 1&1/2 inch groups at 40yds easily. I agree with the post above that when hunting whitetail, you should limit your shot to 40yds at most, usually 25 or under.