I hate the word "best". It doesn't apply to anything in Pokemon. So I'm gonna just write a comparison here. The previous answer is missing details and it didn't consider Seismitoad. Currently there are 6 Pokemon that are Water and Ground type, listed in order of BST below (press the linked dex pages for individual stats):
Swampert (BST: 535)
Seismitoad (BST: 509)
Gastrodon (BST: 475)
Whiscash (BST: 468)
Quagsisre (BST: 430)
1st and 2nd rankings in base stats:
- HP:
- 1: Gastrodon at 111
- 2: Wiscash at 110
- Attack:
- 1: Swampert at 110
- 2: Seismitoad at 95
- Defense:
- 1: Swampert at 90
- 2: Quagsire at 85
- Sp. Attack:
- 1: Gastrodon at 92
- 2: Tied for Swampert and Seismitoad at 95
- Sp. Defense:
- 1: Swampert at 90
- 2: Gastrodon at 82
- Speed:
- 1: Seismitoad at 74
- 2: Tied for Wiscash and Swampert at 60
Source: The Pokemon's linked Dex pages.
If any of this is wrong/outdated, at any time, please mention it in the comments.
Hope I helped! FOR THE FROGS!