What is these days the best for park and ride Bangkok coming from Pattaya by car? Just the airport or are there any better options near BTS?

Can you (legally and safely) leave a car overnight at such places?

Yes there is. Take the elevated highway (past Chonburi) to Bangkok Take the Bang Na exit (you MUST go through toll booths 1-3 on the far left). Drop down to ground level and join the highway. Take the first (or second) slip road to the left and at the traffic lights turn left. You're now on Sukhumvit Road heading south. Pass under Bang Na BTS station and take the second opening on the right. You'll see signs to Manarom hospital. Park up in the road just short of the hospital and walk back to Bang Na BTS (about 5-6 mins). If you have satnav then just type in Manarom hospital and the satnav should do the rest.

Forgot to mention. There are no parking restriction on the road by the hospital and the area is very quiet at night so I don't think your vehicle would be subject to any undue risks.

Another option may be Lad Krabang Station of the Ariport Link, but this will only work if you're there before 8 a.m.

After that time the place is packed and unaccessible.

Take the motorway #7 to Bangkok and take the Airport exit....in the exit to the left, keep right and follow the road for approx. 3 km

Keep right until you see on your right the car park (opposite the AP-link station).

Parking is free, but again.....it'll be packed.

Another if it's for the day is to park inside the MegaMall/Ikea then take a cab to Udom Suk BTS

^There's a regular free bus from magabangna to the BTS.