Best Castle in Lake District

Are you referring to a defensive fortification or a palace/large home? Both types can be referred to as castles in the UK.

I guess it would be a palace/large home.

Wray is the best non-ruined Castle in the Lake District proper (take the boat over from Ambleside). Strictly it was never a Castle, it was a stately home built like a castle, but lets not split hairs.

But spread your wings a bit and it's really easy to get to Carlisle Castle. That is totally the real thing with a very complex history.

Or head out to the West Coast and visit Muncaster Castle. That is best with a car, but is easy enough by public transport. Bus to Workington, train to Ravenglass, then follow the Eskdale walking trail past Walls (the most complete Roman Bath House in the UK) through the woods to Muncaster Castle (30 minutes or so), coming straight back down the hill by the road. On summer weekends some trains are met by a free bus from Ravenglass Station to the Castle.
Splitting hairs again it is a slightly moot point if Muncaster is a Castle or a stately home. In terms of scenic and beautiful my opinion *(as a local) is that Muncaster has it. It also happens to be my closest- I was up there 10 days ago in the gardens. And, TBH, I was at a function with Iona Frost-Pennington a few weeks ago. But beyond that I am not biased, it's just a very small world here.

Another one is Appleby- 45 minutes by car from Keswick or there are 3 buses a day. Again indisputably a real Castle, with a real history.

You would have to be really lucky on your timings and have a car but Rose Castle (former home of the Bishops of Carlisle) would be a good catch. Very infrequently open, your best bet is to go to a function there.

Cockermouth Castle is only open for Heritage Open Days in September.

EDIT- If a large home then Holker Hall (need a car) or Levens Hall (known for its topiary) would be good options (take the 555 bus down south of Kendal).

Wray Parkland and Castle is on the west shore of Windermere. It is a National Trust UK site. The Lake District became popular in 1700s-1800s as a rural nature retreat. The Architecture tends to Georgian. There are several Real Castles in Southwest Scotland. Wales has "zillions" of Castles.

I loved Levens Hall

We do have a car and I did have Levens Hall - mostly for the gardens as there are a couple in my group that enjoy gardens. Appreciate all the advice.

If you have a car (and are happy with the stately home definition of a castle) you can add Sizergh which is only about a mile before Levens Hall.

On the other side of the Highway from the house is Low Sizergh Barn Farm Shop which is rather good (and easily missed if you don't know it is there).

Footpaths in Sizergh Grounds take you up to some very fine limestone escarpment and the remote St John's Church, Helsington. The Church is usually open and has a beautiful painted wall mural on the east wall. It is a war memorial and was funded by Mrs Benson of Levens Hall, frame with the canvas donated by Sir Gerald Strickland of Sizergh Hall. It thus directly links to the day's visits. You can also drive up there, locally it is a popular and wonderful viewpoint.

It was painted in oils (using local children as models for the angels) by Marion de Saumarez (1885-1978) who was the daughter of James, 4th Lord de Saumarez, descendent of a line of officers with distinguished service in the British Navy. She studied painting for 7 years at the Academie Julian, becoming a noted portrait painter. During WW1 she was a nurse with the Red Cross at Shrublands Park Auxiliary Hospital (using one of the family homes). A great surprise in a humble country church.
Today being Armistice Day is an appropriate day to mention it

isn31c - I'm in the tourism industry and work with tour guides everyday. If you're not a tour guide - you should be! :-) You would be great.